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BOTANY 


FOR 


AGRICULTURAL  STUDENTS 


BY 

JOHN   N.   MARTIN 

Professor  of  Botany  at  the  Iowa  State  College  of  Agriculture 
and  Mechanic  Arts 


FIRST    EDITION 


NEW  YORK 

JOHN  WILEY  &  SONS,  Inc. 

London:  CHAPMAN   &  HALL,   Limited 
1919 


Copyright,  1919, 

BY 

JOHN  N.   MARTIN 


Stanhope  ipress 

H.GILSON    COMPANT 
BOSTON,  U.S.A. 


PREFACE 

Although  students  vary  widely  in  their  reasons  for  studying 
Botany,  the  fundamental  facts  or  principles  of  the  subject  are 
not  thereby  altered.  One  has  considerable  freedom,  however,  in 
the  presentation  of  the  subject  to  adapt  the  subject  matter  to 
special  aims  of  different  classes  of  students,  and  especially  is 
■this  true  in  courses  for  agricultural  students,  since  much  of  the 
work  in  Agriculture  is  based  upon  the  principles  of  Botany.  In 
the  choice  of  material  to  illustrate  principles  and  in  the  presen- 
tation of  the  applications  of  principles,  there  is  special  oppor- 
tunity to  relate  courses  in  Botany  to  courses  in  Agriculture. 

In  any  elementary  course  in  Botany,  regardless  of  the  kind  of 
education  the  student  desires  to  obtain,  the  primary  aim  should 
be  to  give  the  student  a  notion  of  the  fundamental  principles  of 
Botany.  This  aim  should  be  the  guiding  one  in  both  recitation 
and  laboratory,  determining  the  trend  of  discussions  in  recita- 
tion, and  the  nature  of  the  material  and  procedure  in  the  lab- 
oratory. The  primary  aim  should  be  accompanied  by  a  secondary- 
aim  to  relate  the  subject  to  the  student's  major  line  of  work. 
When  the  relation  of  the  subject  to  major  lines  of  work  is  obvious, 
the  student  is  more  likely  to' appreciate  the  subject  and  is  thereby 
put  in  a  favorable  mood  to  study  the  subject.  Even  for  students 
who  take  Botany  merely  as  a  part  of  a  general  education,  it  in  no 
way  detracts  from  the  course  or  makes  botanical  training  less 
efficient  to  present  the  practical  aspects  of  the  subject. 

This  book  is  intended  for  elementary  courses  in  Botany  in 
colleges  and  universities.  In  its  preparation  the  aim  has  been  to 
present  the  fundamental  principles  of  Botany  with  emphasis 
upon  the  practical  apphcation  of  these  principles.  The  subject 
matter  is  presented  in  two  parts,  part  I  being  devoted  to  the 
study  of  the  structures  and  functions  chiefly  of  Flowering  Plants, 
and  Part  II,  to  the  study  of  the  kinds  of  plants,  relationships, 
Evolution,  Heredity,  and  Plant  Breeding. 

In  the  preparation  of  the  book,  I  had  the  following  ol)jects  in 
view:    (1)  to  present  the  structures  and  functions  of  Flowering 


86430 


IV  PREFACE 

Plants  and  relate  them  to  such  agricultural  subjects  as  Farm 
Crops,  Forestry,  and  Horticulture,  and  to  the  more  advanced 
courses  in  Botany;  (2)  to  present  the  kinds  of  plants  with  emphasis 
upon  their  evolutionary  relationships  and  their  economic  im- 
portance; and  (3)  to  present  Evolution,  Heredity,  and  Plant 
Breeding  as  related  to  the  improvement  of  plants. 

The  topics  are  arranged  in  the  book  in  the  order  in  which  I 
usually  present  them.  The  presentation  of  the  reproductive 
structures  and  processes  of  Flowering  Plants,  followed  by 
that  of  the  vegetative  organs,  has  fitted  in  at  Iowa  State 
College  with  the  time  of  year  at  which  the  agricultural  students 
begin  the  study  of  Botany  and  also  with  the  courses  in 
Agriculture.  In  other  schools  where  conditions  are  different, 
other  arrangements  of  the  topics  are  more  suitable.  In  recogni- 
tion of  this  fact,  most  of  the  chapters  have  been  written  so  as  to 
be  separately  understandable,  the  aim  being  to  make  the  book 
adaptable  to  any  arrangement  of  topics  that  the  teacher  may 
prefer. 

In  the  discussion  of  a  subject  the  presentation  of  the  general 
features  precedes  that  of  the  particular  features,  and  the  latter 
are  presented  in  most  cases  by  the  study  of  type  plants  chosen  on 
account  of  their  famiharity  and  economic  importance. 

The  book  is  intended  for  an  entire  year's  work  in  Botany  and 
to  be  accompanied  by  laboratory  work.  Where  less  time  is  de- 
voted to  the  subject,  the  organization  of  the  chapters  so  as  to  be 
separately  understandable  permits  a  selection  of  topics  according 
to  the  requirements  of  the  course. 

The  reproductive  structures  and  processes  in  Flowering  Plants 
(Chapters  III  and  IV)  are  dwelt  upon  more  than  is  necessary 
for  students  who  have  had  a  good  course  in  Botany  in  a  high 
school.  A  large  percentage  of  the  students  in  my  elementary 
classes  have  had  no  Botany  and  have  difficulty  in  understanding 
sexual  reproduction  in  Flowering  Plants.  In  an  effort  to  thor- 
oughly acquaint  the  student  with  this  subject,  I  have  dwelt  at 
considerable  length  upon  those  phases  of  the  subject  that  are  in 
my  experience  difficult  for  the  student  to  understand.  In  case 
students  are  familiar  with  this  subject,  parts  of  Chapters  III  and 
IV  can  be  omitted  or  read  hastily  in  review. 

Usually  there  are  some  students  in  the  class  that  are  especially 
interested  in  certain  topics  and  desire  a  more  complete  discussion 


PREFACE  V 

of  the  topics  than  the  text  affords.  In  recognition  of  this  fact, 
I  have  added,  chiefly  as  footnotes,  many  references.  Most  of  the 
references  are  bulletins  on  the  special  topics,  and  in  addition  to 
giving  further  information  on  the  special  topics,  these  references 
introduce  the  student  to  that  vast  source  of  information  contained 
in  the  bulletins  published  by  the  U.  S.  Department  and  the  ex- 
periment stations  of  the  diffejent  states. 

Many  of  the  illustrations  have  been  taken  from  the  publica- 
tions of  various  authors  whose  names  or  the  names  of  their  pub- 
lications appear  in  connection  with  the  illustrations.  To  these 
authors  I  am  much  indebted.  Most  of  the  original  illustrations 
have  been  made  by  Mrs.  Edith  Martin,  who  has  also  given  me 
valuable  assistance  in  other  ways  in  the  preparation  of  the  book. 
Also  much  credit  is  due  Mr.  H.  S.  Doty  and  Mr.  L.  E.  Yocum, 
my  assistants,  who  have  given  me  valuable  suggestions.  To 
Dr.  L.  H.  Pammel,  who  read  some  of  the  topics  on  Fungi  and 
offered  valuable  suggestions,  I  am  also  much  indebted. 

The  book  no  doubt  has  many  faults,  but  I  hope  it  has  some 
particular  value  and  that  the  criticisms  which  teachers  offer  will 
make  me  a  more  efficient  teacher. 

J.  N.  MARTIN. 
Ames,  Iowa 
Oct.  7,  1918 


CONTENTS 


INTRODUCTION 

CHAPTER  PAGE 

I.   The  Nature  and  Subdivisions  of  Botany 1 


II.  A  General  View  of  Plants 


PART  I 

Plants  (Chiefly  Seed  Plants),  as  to  Structures  and  Functions 

III.  Flowers 9 

General  characteristics  and  structure  of  flowers 9 

Some  particular  forms  of  flowers 16 

Arrangement  of  flowers  or  inflorescence 26 

IV.  Pistils  and  Stamens 33 

Structure  and  function  of  pistils  and  stamens      33 

Pollination 46 

V.   Seeds  and  Fruits 55 

Nature  and  structure  of  seeds 55 

Resting  period,  vitality,  and  longevity  of  seeds 67 

Purity  and  analysis  of  seeds 74 

Nature  and  types  of  fruits  of  Flowering  Plants 77 

Dissemination  of  seeds  and  fruits 82 

VI.   Germination  of  Seeds;   Seedlings 89 

Nature  of  germination  and  factors  upon  which  it  depends     .  89 

Germinative  processes 93 

Testing  the  germinatirf^e  capacity  of  seeds 98 

Seedlings     102 

VII.   Cells  and  Tissues     112 

Structure  and  function  of  cells 112 

Respiration 121 

Cell  multiplication 123 

General  view  of  tissues 126 

vii 


viii  CONTENTS 

CHAPTER  PAGE 

VIII.   Roots 135 

General  features  of  roots      135 

Root  structure 143 

Factors  influencing  the  direction  of  growth  in  roots    ....  150 

The  soil  as  the  home  of  roots 152 

Water,  air,  and  parasitic  roots 1G2 

Propagation  by  roots 163 

IX.   Stems 166 

Characteristic  featiu-es  and  types  of  stems 166 

General  structure  of  stems 166 

Structure  of  monocotyledonous  stems 187 

Structure  of  herbaceous  dicotyledonous  stems      192 

Structure  of  woody  stems 197 

X.   Buds:  Growth  of  Stems;  Pruning;  Propagation  by  Stems  204 

Buds 204 

Growth  of  stems 213 

Pruning 221 

Propagation  by  means  of  stems 225 

XI.   Leaves 233 

Characteristic  features  of  leaves      233 

Primary  and  secondary  leaves 234 

General  structure  of  leaves 242 

Cellular  structure  of  leaves 246 

The  manufacture  of  food  by  leaves 252 

Factors  influencing  photosynthesis 257 

Transpiration  from  plants 260 

Respiration 269 

Special  forms  of  leaves      270 

Uses  of  the  photosynthetic  food     273 


PART  II 

Plants  as  to  Kinds,  Relationships,  Evolution,  and  Heredity 

XII.   Introduction 289 

XIII.  Thallophytes 296 

Algae  (Thallophytes  with  a  food-making  pigment)      ....  296 

General  characteristics      296 

Blue-green  Algae  (Cyanophyceae) 297 

Green  Algae  (Chlorophyceae)      301 

Brown  Algae  (Phaeophyceae)      318 

Red  Algae  (Rhodophyceae)      324 

Some  Alga-hlce  Thallophytes  not  definitely  classified      .    .  329 


CONTENTS  IX 


PAGB 


CHAPTER 

XIV.   Thallophytes  {continued) 336 

Myxomycetes   and    Bacteria    (Thallophytes   lacking    food- 
making  pigments) 336 

Myxomycetes  (Slime  Molds) 336 

Bacteria 341 

XV.   Thallophytes  {concluded) 351 

Fungi  (Thallophytes  lacking  food-making  pigments)  ....  351 

Phycomycetcs  (Alga-like  Fungi) 353 

Ascomycetes  (Sac  Fungi)  and  Lichens 363 

Basidiomycetes 382 

Fungi  Imperfect!  (Imperfect  Fungi)       404 

XVI.   Bryophytes  (Moss  Plants) 405 

Liverworts  and  Mosses 405 

Liverworts 406 

Mosses 417 

XVII.   Pteridophytes  (Fern  Plants) 425 

Filicales 426 

Equisetales  (Horsetails) 435 

Lycopodiales  (Club  Mosses) 438 

XVIII.   Spermatophytes  (Seed  Plants) 445 

Gymnosperms  (Seeds  not  enclosed) 445 

Cycads  (Cycadales) 446 

Pines  (Pinaceae) 451 

XIX.   Spermatophytes  {continued)     .  • 459 

Angiosperms  (seeds  enclosed) 459 

XX.   Classification  of  Angiosperms  and  Some  of  Their  Fami- 
lies OF  Most  Economic  Importance 471 

Dicotyledons  (Apetalae) 473 

Dicotyledons  (Polypetalae)      481 

Dicotyledons  (Sympetalae) 489 

Monocotyledons     . 495 

XXI.   Ecological  classification  of  plants ^0*^ 

Nature  of  Ecology ^^^ 

Ecological  factors 501 

Ecological  societies ^04 

Plant  succession 510 

XXII.   Evolution      513 

Meaning  and  Theories  of  Evolution 513 

Experimental  Evolution 524 

XXIII.   Heredity 535 

General  features  of  Heredity 535 

Experimental  study  of  Heredity 537 


CHAPTER 

XXI  V\  Plant  Breeding 


CONTENTS 

PAGE 


657 

Selection 557 

Mass  culture 558 

Pedigree  culture 560 

Selection  of  Mutants 561 

Hybridization 561 

Crossing  and  vigor  of  offspring 564 


BOTANY   FOR   AGRICULTURAL    STUDENTS 
INTRODUCTION 


Botany  for  Agricultural  Students 


CHAPTER  I 
THE  NATURE  OF  BOTANY 

Botany  is  a  branch  of  Biologij  which  includes  all  of  the  sciences 
that  deal  with  living  things.  Zoology,  Bacteriology,  Human 
Anatomy  and  Physiology  are  some  other  biological  sciences  that 
are  familiar  and  closely  related  to  Botany. 

The  word  botany  comes  from  a  Greek  word,  hosko,  meaning, 
"  I  eat.'  Botany  was  originally  the  science  of  things  good  to  eat, 
and  in  its  naming  the  fact  was  recognized  that  plants  are  the 
source  of  our  food.  Of  course  at  the  present  time  Botany  studies 
all  kinds  of  plants  which  include  besides  the  many  useful  for 
food,  many  useful  as  medicine,  and  many  that  are  poisonous. 
Botany  is  commonly  defined  as  that  science  which  treats  of 
plants.  This  definition  is  not  entirely  satisfactory  because  it 
does  not  separate  Botany  from  such  agricultural  subjects  as 
Horticulture,  Forestry,  and  Farm  Crops  which  also  treat  of 
plants. 

Between  Botany  and  those  agricultural  subjects  which  study 
plants,  there  is  no  sharp  division  line.  Much  of  the  work  in 
these  agricultural  subjects  is  based  upon  the  principles  of  Botany. 
Such  features  as  plant  structures,  plant  functions,  and  relation 
of  functions  to  sunlight,  air,  soil,  etc.,  which  are  studied  in  Botany, 
are  features  of  consideration  in  Horticulture,  Forestry,  and  Farm 
Crops.  Although  Botany  and  these  agricultural  subjects  study 
many  plant  features  in  common,  the  latter  subjects  differ  from 
Botany  in  studying  only  special  groups  of  plants,  and  in  limiting 
the  study  to  the  practical  and  economic  phases  of  plants. 

A  plant  may  be  studied  in  a  number  of  different  ways.  It  may 
be  considered  in  reference  to  structure,  functions,  and  in  relation 
to  other  plants.  Botany  is  divided  into  a  number  of  subjects 
which  consider  different  phases  of  plant  life. 

1 


2  THE   NATURE  OF   BOTANY 

Morphology  considers  the  form  and  structure  of  plants.  It 
considers  the  forms  of  plant  bodies  and  the  organs  and  tissues 
which  compose  them.  Morphology  studies  the  structure  of 
roots,  stem,  leaves,  buds,  and  flowers,  and  establishes  the  rela- 
tionships of  organs.  Morphology  not  only  considers  the  more 
complex  plants  but  also  the  simpler  ones,  and  traces  the  develop- 
ment of  plant  structures  through  the  different  plant  groups.  The 
phase  of  Morphology  in  which  the  development  of  the  more 
complex  plants  from  the  simpler  ones  is  studied,  is  called  Plant 
Evolution.  When  Morphology  is  concerned  with  the  micro- 
scopical study  of  the  finer  structures  of  plants,  then  it  is  called 
Anatomy,  and  if  the  study  is  mainly  concerned  with  the  structure 
of  the  cell,  then  it  is  called  Cytology.  Anatomy  and  Cytology  are 
often  spoken  of  as  Histology.  Another  phase  of  Morphology  is 
Embryology  which,  as  the  term  suggests,  is  the  study  of  the 
embrj^o,  or  the  study  of  the  plant  during  its  formation  in  the  seed. 

Plant  Physiology  studies  the  functions  of  plant  structures  and 
the  relation  of  these  functions  to  Hght,  temperature,  air,  soil,  etc. 
It  treats  of  how  the  plant  lives,  respires,  feeds,  grows,  and  re- 
produces. In  the  study  of  Plant  Physiology  we  learn  how  plant 
food  is  made  and  transported,  and  how  plants  grow.  As  a  basis 
for  the  study  of  Plant  Physiology,  one  must  have  a  knowledge 
of  the  Morphology  of  plants  and  also  a  knowledge  of  Chemistry 
and  Physics. 

Plant  Pathology  treats  of  plant  diseases.  In  this  subject  one 
learns  the  disease  producing  plants  and  how  they  affect  the  plant 
diseased.  In  the  study  of  Plant  Pathology,  in  order  to  know  how 
the  diseased  plant  is  injured,  one  must  know  the  nature  and 
function  of  the  tissues  attacked.  This  means  that  one  should 
know  Morphology  and  Plant  Physiology.  Furthermore,  in  order 
to  know  how  the  disease  producing  form  attacks  other  plants  and 
propagates  itself,  one  needs  to  know  its  Morphology  and  Physi- 
ology. 

Plant  Ecology  considers  plants  in  relation  to  the  conditions 
under  which  they  live.  Some  plants  can  live  on  a  dry  hill  top, 
while  others  can  live  only  in  moist,  shady  places.  Some  can  live 
in  colder  regions  than  others.  Some  plants,  like  many  of  the 
weeds,  can  thrive  when  crowded  among  other  plants,  while  some 
like  the  Corn  plant  can  not.  Marshes,  bogs,  forests,  sandbars, 
etc.,  all  have  their  characteristic  plants.     One  set  of  plants  often 


SUBJECTS  TREATED   IN   THIS   BOOK  3 

prepares  the  way  for  others.  On  exposed  rocks  only  very  small 
plants  are  able  to  grow  at  first,  but  due  to  their  presence  soil 
accumulates  and  larger  plants  are  able  to  follow.  Such  problems 
as  the  above  are  studied  in  Ecology.  Ecology  studies  plants  in 
relation  to  the  effects  of  soil,  climate,  and  friendly,  or  hostile 
animals  and  plants.  It  also  studies  the  effect  of  the  different 
conditions  upon  the  form  and  structure  of  plants. 

Plant  Geography  is  much  like  Ecology  and  treats  of  the  dis- 
tribution of  the  different  kinds  of  plants  over  the  earth's  surface. 
Taxonomy,  or  Systematic  Botany,  treats  of  the  classification 
of  plants.  As  a  result  of  this  kind  of  study,  plants  have  been 
arranged  in  groups,  such  as  Algae,  Bacteria,  Fungi,  Mosses, 
Ferns,  and  Seed  Plants.  These  large  groups  are  further  sub- 
divided into  smaller  groups.  Keys  have  been  arranged  by  which 
plants  unknown  to  the  student  may  be  identified.  Through  the 
study  of  Systematic  Botany  one  can  learn  the  names  and  some 
of  the  characteristics  of  the  different  kinds  of  Grasses,  weeds, 
shrubs,  and  trees  that  grow  on  the  farm  or  in  any  other  region. 
Economic  Botany  treats  of  the  uses  of  plants  to  man. 
Paleobotany  is  concerned  with  the  history  of  plants  as  shown 
by  their  preserved  forms,  known  as  fossils,  which  occur  in  the 
different  layers  of  rock  composing  the  earth's  crust.  Paleobotany 
is  studied  in  connection  with  Geology.  In  the  study  of  this 
subject  much  has  been  learned  about  the  plants  which  lived 
millions  of  years  ago,  and  this  knowledge  is  very  useful  in  under- 
standing the  evolution  of  the  plants  which  now  exist. 

Subjects  treated  in  this  Book. — ■  To  become  a  master  in  any 
one  of  the  above  subjects  would  require  years  of  one's  time.  A 
study  of  any  of  the  special  subjects  of  Botany  requires  a  general 
knowledge  of  the  anatomy  and  the  functions  of  plant  struc- 
tures This  means  that  one  must  have  a  general  course  in 
Botany  before  making  a  special  study  of  Morphology,  Plant 
Physiology,  or  any  of  the  special  botanical  subjects.  One 
purpose  of  this  book  is  to  give  a  general  knowledge  of  cultivated 
plants,  of  plants  not  cultivated  but  like  the  Rusts  and  Smuts 
related  to  Agriculture,  and  of  those  plants  which  one  must 
know  in  order  to  understand  the  evolution  of  plants.  Another 
purpose  is  to  give  such  a  general  knowledge  of  plant  anatomy 
and  the  functions  of  plant  structures,  that  one  will  have  the 
necessary  knowledge  for  the  study  of  such  agricultural  subjects 


4  THE   NATURE   OF   BOTANY 

as  Horticulture,  Forestry,  and  Farm  Crops,  and  also  a  basis  for 
the  study  of  the  special  botanical  subjects.  These  special 
subjects  of  Botany  are  not  only  very  important  to  one  who 
makes  a  special  study  of  Botany,  but  some  phases  of  Morphology, 
Plant  Physiology,  Plant  Pathology,  Systematic  Botany,  and 
Ecology  are  important  studies  for  agricultural  students  in  certain 
agricultural  courses.  Part  7,  this  book,  deals  mainly  with  the 
parts  of  plants  as  to  structure  and  function  and,  therefore, 
emphasizes  the  Morphology  and  Physiology  of  plants.  But 
structure  and  function  as  well  as  other  aspects  of  plants,  accom- 
pany and  explain  each  other  and  can  not  well  be  separated  in  an 
elementary  study  of  Botany.  So  the  different  phases  of  the 
plant  are  studied  as  they  occur  in  relation  to  each  other  and 
without  any  designation  as  to  whether  or  not  the  fact  belongs  to 
Morphology,  Physiology,  or  any  other  special  phase  of  Botany. 
Part  II  is  devoted  chiefly  to  a  study  of  plants  as  to  kinds,  rela- 
tionships, evolution,  and  heredity. 


CHAPTER    TI 
A    GENERAL   VIEW    OF   PLANTS 

Abundance  and  Distribution  of  Plants.  —  Plants  are  so  abun- 
dant and  generally  distributed  that  there  are  very  few  regions 
that  do  not  have  plants.  Plants  occur  in  the  water  and  in  the 
soil  as  well  as  on  the  surface  of  the  earth.  Some  plants  live  in 
the  bodies  of  animals.  Some  are  able  to  live  where  the  tem- 
perature is  intensely  cold,  while  others  can  live  in  hot  springs 
where  the  temperature  is  not  far  from  the  boiling  point.  Even 
on  rocks  that  look  quite  bare,  a  close  examination  will  show  that 
some  plant  forms  are  present.  Only  in  exceptional  places,  such 
as  volcanic  regions,  some  hot  springs,  and  regions  of  salt  deposits, 
are  plants  generally  absent. 

The  abundance  or  scarcity  of  plants  in  a  given  region  depends 
upon  how  well  the  conditions  of  the  region  meet  the  requirements 
for  plant  growth.  If  the  soil  is  dry,  as  in  desert  regions,  the 
average  number  of  plants  per  area  is  usually  quite  small,  while  in 
regions  where  there  is  sufficient  moisture  and  sufficient  mineral 
substances,  more  than  100,000  plants  may  occur  on  an  area  no 
larger  than  an  average  garden.  However,  the  number  of  plants 
which  can  occur  on  a  given  area,  is  often  very  different  from  the 
number  that  can  do  well  on  this  same  area.  Many  more  grain 
plants  can  be  grown  per  acre  than  are  grown,  but  agriculturists 
have  learned  that  only  a  limited  number  of  plants  per  acre  can 
do  well.  Among  plants,  as  among  animals,  there  is  competition. 
Plants  must  compete  with  each  other  for  moisture,  mineral 
substances,  and  sunlight,  and  when  the  competition  is  too  great, 
as  occurs  when  plants  are  too  much  crowded,  some  or  all  of  the 
plants  suffer  and  fail  to  produce  good  yields.  By  controlling 
the  amount  of  seed  sown  and  by  properly  distributing  the  seed, 
the  farmer  is  able  to  raise  the  greatest  number  of  plants  per  acre 
with  the  least  loss  from  competition  among  the  plants. 

Diversity  of  Plant  Forms.  —  Plants  are  not  only  the  smallest, 
but  also  the  largest  of  living  organisms.     Many  plants  are  so 


6  A   GENERAL   VIEW   OF   PLANTS 

small  that  they  can  be  seen  on  y  with  a  microscope.  Ranging 
from  these  very  small  plants  to  the  largest  trees,  plants  of  all 
sizes  and  complexity  occur  about  us.  The  different  plant  forms 
differ  very  much  in  structure,  methods  of  getting  food,  and 
methods  of  reproduction.  The  plants  which  concern  us  most 
are  those  which  have  flowers.  They  are  known  as  the  Flowering 
Plants.  Most  of  the  cultivated  plants  and  nearly  all  weeds 
belong  to  this  group.  They  are  the  plants  which  furnish  nearly 
all  of  our  food  and  fibers  and  much  of  our  lumber.  Part  I  of 
this  book  is  devoted  to  the  stud}^  of  the  Flowering  Plants. 

Although  the  Flowering  Plants  concern  us  most,  it  must  not  be 
concluded  that  the  simpler  plants  are  of  no  importance.  The 
simpler  plants,  even  the  microscopic  forms,  not  only  help  and 
hinder  in  the  cultivation  of  the  Flowering  Plants,  but  affect  us  in 
other  ways  and  must  receive  consideration.  Much  of  Part  II 
is  devoted  to  the  study  of  them. 

Parts  of  a  Plant.  —  In  plants,  as  in  animals,  there  is  a  living 
body  consisting  of  parts  each  of  which  has  a  special  work  to 
perform.  The  various  parts  of  a  plant  having  their  own  special 
work  are  called  organs,  and  the  special  work  of  an  organ  is  its 
Junction.  Plants,  like  animals,  being  composed  of  organs,  are 
called  organisms.  In  the  Flowering  Plants,  the  plant  body  con- 
sists of  roots,  stem,  leaves,  buds,  flowers,  seeds,  and  fruit.  All  of 
these  structures  are  not  present  at  all  times,  but  unless  a  Flowering 
Plant  develops  all  of  these  organs  during  its  life,  its  development 
is  considered  incomplete.  Through  the  special  functions  of  its 
organs,  the  plant  is  able  to  exist  and  reproduce  itself.  The  roots 
hold  the  plant  to  the  soil  and  furnish  water  and  salts;  the  stem 
supports  the  leaves,  flowers,  and  fruit  in  the  air  and  sunlight; 
the  leaves  make  food;  the  buds  produce  new  leaves  and  flowers; 
and  the  flowers,  seed,  and  fruit  have  to  do  with  the  production 
of  new  plants.  But  each  organ  is  also  composed  of  parts  and  to 
understand  an  organ  one  must  understand  its  special  groups  of 
cells,  known  as  tissues,  of  which  the  organ  is  composed. 

Life  Cycle  of  Flowering  Plants.  —  A  characteristic  of  living 
organisms  is  their  ability  to  use  substances  as  food,  grow,  and 
develop.  Living  organisms  are  also  much  influenced  by  their 
surroundings.  Plants  are  much  influenced  by  the  nature  of  the 
soil,  air,  sunlight,  and  plants  which  grow  about  them. 

To  understand  a  plant  one  needs  to  study  it  in  its  various 


LIFE  CYCLE  OF   FLOWERING   PLANTS  7 

stages  of  development.  The  tiny  Corn  plant,  called  embryo  or 
germ,  which  we  find  in  the  Corn  kernel,  does  not  look  much  like 
the  plant  that  bears  tassel  and  ears.  From  the  embryo  to  the 
flower  and  seed  stage,  many  things  take  place.  The  series  of 
events  which  take  place  in  the  development  of  the  embryo  to  a 
mature  plant  constitutes  the  life  cycle  of  a  plant.     Starting  from 


Fig.  1.  —  Life  cycle  as  illustrated  by  the  Corn  plant,  a,  mature  kernel; 
b,  germination;  next,  seedling;  d,  mature  plant  composed  of  roots,  stems, 
leaves,  and  flowers,  all  of  which  are  composed  of  tissues  having  special  func- 
tions to  perform;  e,  the  two  kinds  of  flowers  with  pollination  indicated; 
f,  fertiUzation  indicated  by  the  two  globular  bodies,  sperm  and  egg,  on  the 
inside  of  the  ovary  or  portion  that  develops  into  the  kernel.  After  ferti- 
lization the  ovary  develops  into  another  kernel  and  thus  the  life  cycle  is 
completed. 


the  seed,  this  series  of  events  consists  of  germination,  develop- 
ment of  seedling  with  its  different  organs  and  tissues,  develop- 
ment of  root,  stem  bud,  and  leaf  structures  of  the  more  mature 
plant,  development  of  flowers,  pollination  and  fertilization,  and 
development  of  other  kernels.  The  life  cycle  of  any  Flowering 
Plant  is  similar  to  that  of  the  Corn.    Thus  it  is  seen  that  the  life 


8  A  GENERAL   VIEW  OF   PLANTS 

cycle  of  a  plant  returns  us  to  the  place  of  starting.  The  series  of 
events  may  be  represented  as  shown  in  Figure  1,  and  in  tracing 
them  one  can  begin  at  any  point.  The  yield  of  the  plant  at 
maturity  depends  upon  how  well  it  has  done  at  the  different 
stages  in  its  life  cycle.  The  purpose  of  cultivation  is  to  help  the 
plant  to  do  well  at  all  stages,  and  it  is  for  this  reason  that  we  look 
after  the  fertility  of  the  soil,  select  seed,  prepare  a  seed  bed,  sow 
or  plant  a  certain  amount  of  seed  and  in  a  certain  way,  prevent 
the  growth  of  weeds,  etc.  But  often  methods  of  cultivation 
must  take  into  account  the  structure  and  function  of  plant 
organs  as  they  occur  at  the  different  stages  in  the  life  cycle  of 
the  plant,  and  unless  the  peculiar  features  of  the  plant  are 
understood,  the  methods  employed  in  cultivation  may  not  be 
adapted  to  secure  the  best  results. 


PART   I 

PLANTS   (CHIEFLY  SEED  PLANTS)   AS  TO  STRUC- 
TURES AND  FUNCTIONS 


CHAPTER   III 

FLOWERS 

General  Characteristics  and  Structure  of  Flowers 

On  account  of  their  colors  and  odors,  flowers  very  much  excel 
other  plant  organs  in  attracting  attention.  Everybody  is  in- 
terested in  flowers  on  account  of  their  sesthetic  charm,  if  for  no 
other  reason.  The  attractive  colors  and  pleasant  odors  common 
to  flowers  not  only  interest  the  scientist  but  also  appeal  to  the 
sesthetic  sense  of  people  in  general.  In  fact  many  people  would 
define  the  flower  as  the  showy  part  of  the  plant.  However 
showiness  is  not  an  essential  feature,  for  there  are  many  flowers 
which  have  no  attractive  colors  or  odors  and  yet  they  are  just  as 
genuine  in  function  as  are  showy  flowers.  Most  forest  and  shade 
trees,  the  Grasses,  and  many  weeds  do  not  have  showy  flowers. 
The  flowers  of  such  plants  as  the  Oaks,  Elms,  Maples,  and  Pines 
lack  showy  parts  and  are  so  inconspicuous  that  most  people  have 
not  noticed  them,  yet  these  flowers  are  just  as  genuine  in  function 
as  those  of  a  Lily  or  Rose. 

On  account  of  their  showiness  and  importance  in  reproduction, 
flowers  were  first  to  receive  careful  study;  and  in  the  early 
history  of  Botany,  flowers  were  about  the  only  plant  structures 
that  received  much  attention.  At  the  present  time  there  are 
some  people  who  have  the  erroneous  notion  that  the  study  of 
Botany  and  flowers  are  still  almost  identical  despite  the  fact  that 
the  study  of  flowers  is  now  of  no  more  importance  than  many 
other  phases  of  plant  life,  as  is  well  shown  by  the  large  amount 
of  space  devoted  by  our  present  botanical  texts  to  the  study  of 
roots,  stems,  leaves,  and  other  phases  of  plants. 

In  size,  flowers  may  be  almost  microscopical  as  in  some  of  the 
small  floating  water  plants,  such  as  the  Duckweeds,  or  they  may 
be  of  huge  dimensions  as  some  tropical  flowers  which  are  two  or 
more  feet  across.  Even  in  the  ordinary  greenhouse,  some  flowers 
are  so  small  that  they  are  not  conspicuous  except  in  large  clusters, 

9 


10  FLOWERS 

while  those  of  Carnations  and  Roses  are  conspicuous  when  single. 
In  Chrysanthemums,  Daisies,  and  Sunflowers  the  individual 
flowers,  although  small,  form  a  cluster  so  compact  that  it  is  often 
erroneously  considered  a  single  flower. 

As  to  color,  which  is  the  character  most  closely  related  to 
securing  pollination  by  insects,  flowers  are  exceedingly  various. 
Some,  especially  those  that  depend  upon  the  wind  for  pollination, 
are  green  like  leaves.  Some  are  white,  while  among  others  nearly 
every  color  imaginable  can  be  found.  It  is  claimed  that  b}^  means 
of  colors  flowers  solicit  the  visitation  of  insects,  which  are  im- 
portant agents  in  pollination. 

The  odors  of  flowers,  usually  pleasant,  but  sometimes  repul- 
sive to  us,  as  in  case  of  the 
Carrion-flower  and  Skunk 
Cabbage,  probably  serve  in 
attracting  insects.  Fm-ther- 
more,  pleasant  odors  add 
P  ---^^^^^^     )^^^^<^y''^  ^^  ^^^  value  of  plants  for 

ornamental  purposes. 

Flowers  present  various 
forms.  When  well  open, 
Fig.  2.  -  Basswood  flower  with  portions  some  are  wheel-shaped, 
removed  from  one  side  so  that  the  interior  some  funnel-shaped,  some 
of  the  flower  may  be  seen,  a,  calyx  com-  tubular,  while  others  de- 
posed of  leaf-hke  portions  or  sepals;  o,  p^rt  from  these  forms  with 
corolla  composed  of  leaf-like  portions  called  ^^^-^^^  irregularities,  as  in 
petals;   s,  stamens;   p,  pistil;   r,  receptacle.      ,       ^  "  ,  , 

Much  enlarged.  tlie  Sweet  Pea,  where  the 

flower  resembles  a  butter- 
fly in  shape,  or  in  the  Orchids  where  parts  of  the  flower  may  be 
so  shaped  as  to  resemble  a  slipper,  as  the  Orchid  known  as  the 
Lady's-slipper  illustrates.  The  shape  of  the  flower  in  many  cases 
favors  the  visitation  of  only  special  insects,  and,  therefore,  is 
closely  related  to  the  problem  of  pollination. 

To  discover  the  essential  features  of  a  flower,  it  becomes 
necessary  to  determine  the  function  of  the  flower,  and  become 
acquainted  with  its  parts  and  the  use  of  each  part  in  relation  to 
the  work  of  the  flower. 

Function  of  the  Flower.  —  The  flower  is  the  plant's  principal 
organ  of  reproduction,  being  devoted  to  the  production  of  seed 
which  is  the  plant's  principal  device  for  producing  new  plants. 


PARTS  OF  THE   FLOWER  11 

Functionally,  the  flower  may  be  defined  as  the  organ  which  has 
to  do  with  seed  production.  Flowers  which  have  been  so  modi- 
fied through  cultivation  that  they  no  longer  produce  seed  are  not 
true  flowers.  However,  the  true  function  of  the  flower  is  often 
not  the  important  feature  to  the  plant  grower.  Many  flowers 
are  cultivated  entirely  for  their  sesthetic  charm.  In  case  of 
fruit  trees,  Tomatoes,  and  many  other  plants,  the  structure 
developing  from  the  flower  and 

known    as    the    fruit    is    more  /'\ 

important  to  the  plant  grower      r      ^-^  4^  i  qS^  K    ' 
than  the  seed.     However,  when       \   VnnA^QIi''^^  > 

plants   are    grown   for    seed   or         \     ^i^iyj^A^ 
fruit,  the  amount  of  seed  or  fruit  ^^ji^^^W  ,'|  Wfy^^='^ 

harvested   depends   very    much      /^^^^^M^^^^^^^'^^ 
upon  the  number  of  flowers  pro-    /    ^^    /^  W^  v  ^'''     T~ 
duced.      For  example,  the  gar-  ,^^_^ —  ^\  *"      >;x    y    I 

dener  does  not  expect  to  gather  x_y 

many    Beans    or    Peas    if    the      Fig.  3. -Apetalous  flower  of  Buck- 
,  1        r       xj  wheat,     c,  calyx;  s,  stamens;  p,  pistil; 

vmes  produce  only  a  few  flowers,  ^^^^^^p^^^j^     Much  enlarged.    After 
Likewise,  good  crops  of  Clover  Marchand. 
and  Alfalfa  seed  depend  upon  a 

good  crop  of  flowers;  and  not  much  fruit  is  expected  when  the 
flowers  in  the  orchard  are  few.  It  is  in  connection  with  the 
function  of  reproduction,  that  flowers  have  developed  the  various 
colors,  forms,  and  odors  which  assist  in  bringing  about  fertiliza- 
tion, the  central  feature  of  sexual  reproduction  to  which  the 
flower  is  devoted,  and  the  process  upon  which  the  development 
of  seed  usually  depends. 

Despite  the  multitudinous  forms  and  colors  which  flowers 
present,  there  is  much  unity  and  simplicity  in  structure,  all  parts 
being  organized  to  assist  in  performing  the  function  of  seed 
production. 

Parts  of  the  Flower.  —  The  parts  of  a  flower  are  of  two  general 
kinds;  those  which  are  directly  concerned  in  the  production  of 
seed;  and  those  which  act  as  protective  and  attractive  organs. 
The  former  are  known  as  the  essential  organs,  and  consist  of 
stamens  and  pistils.  The  latter  are  known  as  floral  envelopes 
or  perianth,  and  usually  consist  of  two  sets  of  organs,  one  called 
cahjx  and  the  other,  corolla.  In  Figure  2,  the  calyx  is  the  lowest 
whorl  and  consists  of  green  leaf-like  portions  called  sepals.     The 


12 


FLOWERS 


second  whorl  is  the  corolla  and  each  separate  portion  is  a  petal. 
The  pistil  occupies  the  central  position  and  is  surrounded  by  the 
whorl  of  stamens.     The  end  of  the  flower  stem  to  which  these 


-C 


Fig.  4.  — A  flower  of  Tobacco.   c,the         Fig.  5.  —  Flower  of  Red  Clover, 

funnel-shaped  corolla  made  up  of  united  c,  corolla;   6,  cup-like  calyx.      Much 

petals;    h,  calyx.     The  sepals  are  also  enlarged.     After  Hayden. 
united  below.     Reduced. 

floral    parts    are    attached   is    called    torus    or    receptacle.     The 
receptacle  may  be  flat,  conical,  or  cup-shaped,  and  often  forms 


A  B 

Fig.  6.  —  The  two  unisexual  flowers  of  the  Pumpkin  with  a  portion  of 
the  bell-shaped  coroHas  torn  away  to  show  the  interior  of  the  flowers. 

A,  staminate  flower;  s,  stamens  fitting  together,  forming  a  column.  B, 
pistillate  flower.     Less  than  half  natural  size. 

an  important  part  of  the  fruit.  The  corolla  is  usually  bright 
colored,  and,  therefore,  the  conspicuous  part  of  the  flower.  It  is 
also  the  fleeting  part  of  the  flower,  usually  lasting  only  a  few  days. 


UNISEXUAL   FLOWERS 


13 


Fig.  7.  —  Section 
through  a  flower  of  the 
Peach.  There  is  but 
one  pistil  (p),  but  many 
Much  en- 


Flowers  having  the  four  sets  of  organs,  as  shown  in  Figure  2, 
are  called  complete  flowers  to  distinguish  them  from  incomplete 
flowers,  that  is,  flowers  in  which  some  of  the 
organs  are  lacking.  The  organs  are  gener- 
ally arranged  in  a  circular  fashion  around 
the  receptacle,  and  are  characterized  as  be- 
ing in  cycles  or  whorls.  In  some  flowers  a 
part  or  all  of  the  perianth  is  lacking.  In 
the  Buckwheat,  as  shown  in  Figure  3,  only 
one  whorl  surrounds  the  stamens  and  pistil, 
and  it  is  evident  that  this  flower  does  not 
have  both  calyx  and  corolla.  In  such  cases, 
the  petals  are  considered  missing  and  the 
flower  is  said  to  be  apetalous  ("  without 
petals")-  Often  instead  of  being  composed 
of  entirely  separate  petals  (polypetalous), 
the  corolla  is  a  tube  or  funnel-shaped  struc- 
ture, which  appears  to  be  composed  of  united  stamens  (s 
petals  (gamopetalous),  separate  only  at  the  ^^^S^'^^- 
top.     {Fig.  4-)     The  flowers  of  the  Tobacco  Plant,  Pumpkins, 

Squashes,    and    Water- 
melons are  examples  of 
gamopetalous      flowers. 
In  some  cases,  as  in  the 
Tobacco,     Clover,     and 
some  other  plants,   the 
sepals    seem     to     have 
joined  into  one  structure 
(gamosepalous),  forming 
a  tube-  or  cup-like  calyx. 
(Fig.  4  and  5.)     Flowers 
also  differ  in  the  essential 
organs  contained. 
Fig.  8.  —  Section  through  an  Apple  flower        Unisexual  Flowers.  — 
showing  the  compound  pistil  composed  of  five    Flowers  having  both  sta- 
carpels.     The  five  carpels  (a)  are  free  above    mens     and     pistils      are 
corolla;  s,  stamens;  i,   \,nown  as  perfect  or  bisex- 
ual   flowers.     In     some 
plants,  the  stamens  and  pistils  occur  in  different  flowers,  in  which 
case  the  flower  having  stamens  only  is  called  a  staminate  flower, 


but  joined  below,      c, 
calyx.     Much  enlarged 


14 


FLOWERS 


while  the  other  having  pistils  only  is  called  a  pistillate  flower.  Such 
flowers  are  said  to  be  unisexual.  Pumpkins,  Cucumbers,  Corn, 
Hemp,  Willows,  and  Poplars  are  some  of  the  familiar  plants 
which  have  unisexual  flowers.  In  Figure  6  are  shown  the  uni- 
sexual flowers  of  the  Pumpkin.  In  some  cases,  as  in  Corn, 
Cucumbers,  and  Pumpkins,  both  staminate  and  pistillate  flowers 

are  borne  on  the  same  plant. 
Such  plants  are  said  to  be  monw- 
cious  (meaning  "  of  one  house- 
hold ").  In  other  cases,  as  in 
Hemp,  Willows,  and  Poplars, 
the  staminate  and  pistillate 
flowers  are  borne  on  different  in- 
dividuals, that  is,  one  plant  has 
Fig.  9.— Section  through  the  flower  only  staminate  while  another  has 
of  Cotton,    s,  stamens  joined  into  a  only    pistillate    flowers.      Such 


tube  which  surrounds  the  pistil;  p, 
pistil  composed  of  carpels  more  united 
than  those  of  the  Apple.  Smaller 
than  natural  size.     After  Baillon. 


plants  are  said  to  be  dicecious 
(meaning  "  of  two  households  ")• 
Pistils  and  Stamens.  —  As 
everyone  knows,  the  pistils  are 
the  organs  in  which  fertilization  occurs  and  seed  is  produced,  while 
the  stamens  furnish  the  pollen,  which  is  essential  for  fertilization. 
Flowers  usually  have  more  stamens  than  pistils,  but  the  number 


Fig.  10.  —  A  flower  of  a  Legume  with  petals  removed  to  show  the  dia- 
delphous  stamens,  a,  free  stamen;  b,  tube  formed  by  the  joining  of  the 
other  stamens. 


of  each  varies  much  in  the  flowers  of  different  plants.  Some 
flowers,  as  those  of  the  Strawberry,  have  numerous  stamens  and 
pistils,  while  in  some  flowers,  as  in  the  Peach  or  Plum,  there  is 
only  one  pistil,  but  many  stamens.  {Fig.  7.)  The  Apple  flower, 
which  has  many  stamens,  really  has  five  pistils,  but  the  lower 
parts  of  the  pistils  are  joined,  leaving  only  the  upper  parts  free. 


PISTILS  AND   STAMENS 


15 


A  pistil  like  that  of  the  Apple  is  called  a  compound  pistil, 
and  the  pistil-hke  structures  which  compose  it,  instead  of  being 
called  pistils,  are  called  carpels.  Thus  in  Figure  8,  each  of  the 
branches  in  the  upper  region  of  the  pistil  is  the  upper  portion  of 
a  carpel.  If  the  enlarged  bases  of  these  were  separated,  then 
each  carpel  would  resemble  the  pistil  of  the  Cherry  or  Plum 
flower.  Pistils  like  those  of  the  Cherry  and  Plum  consist  of  only 
one  carpel  and  are,  therefore,  called  simple  pistils.  In  flowers 
having  but  one  carpel^  pistil  and  carpel  mean  the  same  thing.  The 
flower  of  the  Cotton  Plant,  shown  in  Figure  9,  has  a  compound 
pistil  in  which  the  carpels  are  more  united  than  in  the  Apple. 

In  most  flowers  the  stamens  are  separate  from  one  another 
(polyadelphous),  but  in  some  groups  of  plants  they  are  more  or 


Fig.  11.  —  A,  hypogynous  flower  of  Piiik;   B,  perigynous  flower  of  Cherry; 
C,  epigynous  flower  of  Wild  Carrot.     Modified  from  Warming. 


less  united  (monadelphous).  In  Cotton  and  other  plants  of  this 
group,  the  stamens  are  joined  in  such  a  way  as  to  form  a  tube 
around  the  pistil.  (Fig.  9.)  In  Clover,  Alfalfa,  and  some  other 
plants  of  this  family,  the  ten  stamens  form  two  groups  (diadel- 
phous),  nine  being  joined  and  one  remaining  free. 

The  relative  positions  of  the  different  parts  of  the  flower  show 
considerable  variation.  In  some  flowers,  as  those  of  the  Dande- 
lion or  Sunflower  illustrate,  the  calyx,  corolla,  and  stamens  arise 
from  the  top  of  the  ovary.  {Fig.  24-)  Such  flowers  are  epigy- 
nous, i.e.,  the  floral  structures  are  on  the  gynous  the  word 
"  gynous  "  referring  to  the  ovary,  which  in  this  case  is  described 
as  inferior.  In  the  Basswood  flower,  calyx,  corolla,  and  stamens 
are  attached  to  the  receptacle  at  the  base  of  the  ovary,  which  is 


16 


FLOWERS 


described  as  superior.  Such 
flowers  are  hypogynous.  In 
some  flowers,  as  in  the  Peach 
shown  in  Figure  7,  the  calyx, 
corolla,  and  stamens  are  at- 
tached to  the  rim  of  a  cup- 
like structure  surrounding  the 
ovary.  In  this  case  the  flower 
is  perigynous,  and  the  ovary 
is  described  as  half  inferior.  To 
which  of  the  above  classes  does 
the  Apple  flower  belong?  In 
Figure  11  the  three  positions 
of  the  perianth  and  stamens  in 
reference  to  the  ovary  are  shown 
for  comparison. 

Some  Particular  Forms  of 
Flowers 

That  there  are  numerous 
differences  among  flowers  is 
shown  by  the  fact  that  largely 
upon  differences  pertaining  to 
flowers,  the  Flowering  Plants 
have  been  divided  into  many 
classes,  such  as  orders,  which  in 
turn  are  subdivided  into  fami- 
lies, then  into  genera,  and 
finally  into  species  of  which 
there  are  more  than  100,000. 
The  differences  are  mainly  struc- 
tural, and  between  flowers  of 
Fig.    12.  —  Corn   plant,     t,   tassel     ■.-n-         ,   e       -t       .i  r. 

.    ^     •    ^     a  different  families  they  are  often 

consisting   ot    staminate   flowers;    e,         . 

ears  on  which  the  pistillate  flowers  ^uite  prominent.     For  example, 
are  found.  when  such  flowers  as  those  of  the 

Grass,  Bean,  Sunflow^er,  and 
Orchid  family  are  compared,  that  there  are  peculiar  differences 
in  the  character  of  flowers  is  obvious. 

Grass  Flowers.  —  One  of  the  characteristic  features  of  the 
Grass  flowers  is,  that  there  are  no  showy  organs.     Grass  flowers 


(X)llN   FLOWERS 


17 


of    such    Grasses 


-___>/> 


are  usually  green  like  leaves,  and  their  stamens  and  pistils  are 
enclosed  and  protected  by  small  leaf-like  bodies  called  bracts, 
which  take  the  place  of  a  calyx  and  corolla.  Although  quite 
inconspicuous,  yet  in  being  characteristic 
as  Corn,  Wheat,  Oats,  Barley,  Rye,  Rice, 
and  Timothy,  Grass  flowers  are  so  im- 
portant that  they  deserve  some  special 
attention. 

Corn  Flowers.  —  As  alread}^  stated  (page 
14)  Corn  flowers  are  unisexual.  The  stami- 
nate  flowers  are  produced  in  the  tassel, 
while  the  pistillate  flowers  occur  on  the  ear. 
(Fig.  12.) 

The  staminate  flowers  bear  three  stamens 
and  occur  in  groups  of  twos,  called  spikelets. 
The  branches  of  the  tassel  upon  which  the 
spikelets  are  crowded  are  known  as  spikes. 
In  Figure  13  is  shown  a  spike  or  branch  of 
the  Corn  tassel  so  drawn  as  to  show  the 
spikelets. 

The  two  flowers  of  each  spikelet  are  in  such 
close  contact,  that  in  order  to  identify  each 
flower,  the  bracts  must  be  spread  apart  as 
shown  in  Figure  14-  In  the  older  flower,  the 
stamens  have  elongated  and  pushed  out  of 
the  bracts.  The  boat-shaped  bracts  are  so 
fitted  together  as  to  make  a  good  enclosure  for  the  stamens 
during  their  development.  The  two  outer  bracts,  situated 
on  opposite  sides  of  the  spikelet  and  facing  each  other,  so  as 
to  close  together  and  enclose  the  flowers,  are  known  as  glumes. 
Between  each  glume  and  set  of  stamens  is  the  bract  called  lemma. 
The  bract  on  the  opposite  side  of  the  stamens,  with  its  concave 
side  turned  toward  that  of  the  lemma,  is  known  as  the  palea. 
The  palea  and  lemma,  when  closed  against  each  other,  enclose 
the  stamens.  The  small  bodies  at  the  base  of  the  stamens  are 
called  lodicules,  and  may,  by  their  swelling,  spread  the  bracts 
apart,  thus  helping  the  stamens  to  escape  from  their  enclosure. 
The  structure  of  the  flower  will  be  more  easily  understood  by  a 
study  of  Figure  14-  The  glume  is  not  considered  a  part  of  the 
flower.     The  two  glumes  form  a  covering  for  the  spikelet. 


Fig.  13. — A  branch 
or  spike  from  the 
Corn  tassel,  sp, 
spikelets.  Only  three 
of  the  spikelets  are 
pointed  out.  Slightly 
enlarged. 


18 


FLOWERS 


Other  names  are  often  applied  to  the  glume  and  lemma.  In 
courses  in  Agriculture,  the  glume  is  often  called  outer  or  empty 
glume  and  the  lemma,  the  flowering  glume. 

The  pistillate  flowers  are  arranged  on  a  cob  and  enclosed  by 
husks,  so  that  only  the  outer  ends  or  silks  of  the  pistils  are 


Fig.  14.  —  A  spikelet  from  the  Corn  tassel 
two  staminate  flowers 

The  flowers  are  numbered  (1)  and  (2),  No.  1  being  more  mature 
/,  lemma;  /;,  palea;  s,  stamens;  I,  lodicules. 


Much  enlarged  to  show  the 
glumes; 


exposed.  When  the  husks  are  removed,  the  flowers  are  seen 
arranged  on  the  cob  just  as  the  kernels  are  in  the  mature  ear,  for 
each  kernel  develops  from  a  flower.  Explain  what  is  shown  in 
Figure  15.  The  pistillate  flowers  occur  in  groups  of  two's  or 
spikelets,  but  only  one  flower  of  the  spikelet  completes  its 
development.  The  flower  which  remains  rudimentary  develops 
no  silk  and  remains  so  inconspicuous  that  one  needs  a  magnifier 
to  see  it.  Since  it  has  no  pistil,  its  presence  is  known  only  by  its 
bracts.  In  Figure  16,  point  out  the  rudimentary  flower  and  the 
one  that  develops. 


CORN   FLOWERS 


19 


Fig.  1").  —  Lengthwise  section 
through  the  end  of  a  young  ear 
of  Corn,  showing  the  spikelets 
containing  the  pistillate  flowers. 
h,  husk;  s,  silks  of  the  pistils;  b, 
enlarged  bases  of  the  pistils  en- 
closed by  bracts;  c,  cob.  Slightly 
enlarged. 


Fig.  1G.  —  A  spikelet  from  a  young 
ear  of  Corn  to  show  the  two  pistil- 
late flowers.  /,  the  bracts  of  the 
flower  that  develops  no  pistil.  The 
other  bracts  belong  to  the  flower 
having  the  pistil,  r,  ovary  which 
becomes  the  kernel;  t,  style  of  the 
silk;  s,  the  branched  stigma;  e, 
glumes;  /,  lemmas;  pa,  paleas. 
The  lodicules  are  very  small  and  are 
not  shown.     Very  much  enlarged. 


20 


FLOWERS 


A  study  of  Figure  16  shows  that  the  base  of  the  pistil  is  sur- 
rounded by  bracts,  corresponding  to  those  surrounding  the 
stamens  in  the  staminate  flowers.  The  bracts  of  the  pistillate 
flowers  are  small,  membranous,  and  form  the  chaff  of  the  cob. 

Oat  Flower.  —  A  head  of 
Oats,  as  shown  in  Figure  17,  is 
much  branched  and  the  spike- 
lets  occur  at  the  ends  of  the 
branches.  Each  spikelet  con- 
sists of  two  or  more  flowers, 
which  are  well  enclosed  by  the 
two  glumes.  When  the  glumes 
are  spread  apart  as  shown  in 
Figure  18,  it  is  seen  that  the 
flowers  are  attached,  one  above 
another,  to  a  small  slender  axis. 
This  axis  is  known  as  the  ra- 
chilla.  Rachilla  means  small 
rachis."  Rachis  is  the  name 
applied  to  the  main  axis  of 
the  Oat  head  from  which  the 
branches  arise.  The  small 
branches  bearing  the  spike- 
lets  at  their  ends  are  called 
pedicels.  Thus  branches  arise 
from  the  rachis  and  end  in 
the  rachilla  to  which  the 
flowers  of  the  spikelets  are 
attached. 
The  spikelet  shown  in  Figure  18  contains  three  flowers,  but 
the  upper  one  is  rudimentary  and,  therefore,  produces  no  grain. 
There  is  one  very  important  difference  between  the  flowers  of 
Oats  and  those  of  Corn.  In  Corn  the  pistils  and  stamens  occur  in 
different  flowers,  but  in  Oats  the  stamens  and  pistils  occur  to- 
gether in  the  same  flower.  The  Oat  flower  is,  therefore,  a  perfect 
or  bisexual  flower.  In  each  Oat  flower  there  is  one  pistil  and 
three  stamens  enclosed  by  the  lemma  and  palea.  The  lodicules, 
which  are  two  small  scale-like  bracts  at  the  base  of  the  pistil  and 
stamens,  are  not  easily  seen  in  the  Oat  flower.  The  two  glumes 
of  the  Oat  spikelet  are  so  large  that  when  closed  together  they 


Fig.  17.  —  Head  or  panicle  of  the 
Oat  plant,  s,  spikelets;  6,  branches; 
r,  rachis;  p,  pedicels.  About  one-half 
natural  size. 


21 


Fig.  18.  —  Spikelet  of  the  Oat  head,  with  the  bracts  spread  apart  to  show 
the  flowers.  There  are  three  flowers,  only  (1)  and  (2)  of  which  develop  and 
produce  kernels,  e,  glumes  or  empty  glumes;  /,  lemma  or  flowering  glume; 
pa,  palea;  s,  stamens;  p,  pistil;  r,  rachilla.  The  parts  of  flowers  (2)  and 
(5)  are  not  indicated.     Many  times  enlarged. 


Fig.  19.  —  Two  views  of  a  head  of  Wheat  with  some  spikelets  removed 
to  show  the  zig-zag  rachis.  An  edge  view  of  the  spikelets  is  shown  at  the 
left  and  a  side  view  at  the  right,     r,  rachis;  s,  spikelets. 


22 


FLOWERS 


almost  completely  enclose  the  flowers  of  the  spikelet.  In  thresh- 
ing most  varieties  of  Oats,  only  the  glumes  are  removed,  the 
kernel  still  remaining  enclosed  by  the  lemma  and  palea,  which 
form  the  covering  known  as  the  hull  of  the  grain.  A  grain  of 
Oats,  therefore  consists  of  the  kernel  and  its  hull;  and  the 
quality  of  Oats  depends  much  upon  the  proportion  of  hull  to 
kernel.      As   indicated   in   Figure   18,    the   lower   flower   grows 


Fig.  20.  —  Spikelet  of  Wheat  much  enkrged  and  shown  with  the  bracts 
spread  apart,  so  that  parts  of  the  flower  may  be  seen.  The  flowers  are  num- 
bered and  the  parts  of  one  flower  are  labelled,  e,  outer  glumes;  /,  lemma; 
ya,  palea;  p,  pistil;  s,  stamens;  I,  lodicule;  a,  awn  or  beard;  r,  rachis. 


more  rapidly  than  the  others  and  forms  the  larger  kernel  to 
which  the  smaller  one  sometimes  remains  attached  after 
threshing. 

Wheat  Flowers.  —  In  Wheat  the  head,  usually  called  spike, 
consists  of  many  spikelets  arranged  in  two  rows  along  the  zig-zag 
axis  of  the  head.  {Fig.  19.)  This  zig-zag  axis  is  the  rachis  of 
the  spike.     The  spikelets  are  not  borne  at  the  ends  of  branches 


FLOWERS   OF   THE   LEGUMES   OR   BEAN   FAMILY 


23 


as  in  Oats,  but  are  directly  attached  to  the  rachis.  This  feature 
distinguishes  the  spike  from  the  branching  head,  called  panicle, 
of  the  Oats.  In  the  varieties  of  common  Wheat,  each  spikelet 
contains  three  or  more  flowers  arranged  one  above  another  on  the 
racliilla,  and  one  or  more  of  the  upper  flowers  are  rudimentary. 
Each  fully  developed  flower,  just  as  in  Oats,  consists  of  three 
stamens  and  a  pistil  enclosed  by  the  lemma 
and  palea.  The  lodicules,  like  those  of  the 
Oat  flower,  are  small  inconspicuous  scales  at 
the  base  of  pistil  and  stamens.  In  Wheat, 
where  the  spikelets  are  broad,  the  spikelet  is 
only  partly  enclosed  by  the  glumes.  In  thresh- 
ing Wheat  the  kernel  is  separated  from  the 
bracts  —  the  latter  being  blown  away  as  chaff. 
A  study  of  the 
spikelet  shown  in 
Figure  20  will  aid 
the  student  in  un- 
^"^  '"''  "'"  ''  derstanding  the 
tructure  of 
Wheat  flowers 
and  their  arrange- 
ment in  the  spike- 
let. ^r  y'  ^' 
Flov^fers  of  the 
Legumes  or  Bean 
Family.— The 
flowers  of  the 
Bean  Family  of 
which  Beans, 
Peas,  Clover,  Al- 
falfa,   and   Vetch 

are  familiar  representatives  have  a 
number  of  peculiar  features.  The 
one  most  prominent  among  the 
cultivated  ones  of  the  family  is  the 
irregularity  in  the  shape  of  the  parts  of  the  perianth,  as  the 
flowers  of  Peas  or  Red  Clover  illustrate.  The  calyx  is  a  shallow 
five-toothed  cup.  The  corolla  is  composed  of  four  pieces;  the 
large  expanded  portion  at  the  back,  known  as  the  standard  or 


Fig.  22.  —  End  view  of  an  iiij- 
tripped  and  tripped  flower  of  Red 
Clover. 

h,  flower  untripped.  a,  stand- 
ard; w,  wings;  h,  keel,  d,  flower 
tripped,  in  which  case  the  keel  and 
wings  are  bent  down,  exposing  the 
pistil  (p)  and  stamens  («).  Much 
enlarged.     After  C.  M.  King. 


Fig.  2L— Flower 
of  Red  Clover,  ca, 
calyx;  co,  corolla; 
a,  standard;  w, 
wings;  k,  keel. 
Many  times  en- 
larged. After  C. 
M.  King. 


24 


FLOWERS 


hanticr;  the  two  side  pieces,  known  as  wings;  and  the  single 
boat-shaped  portion  beneath  the  wings,  known  as  the  keel.  In 
the  Red  Clover  flower  shown  in  Figure  21,  these  parts  are  pointed 
out.  The  stamens  and  pistil  are  entirely  enclosed  by  the  keel, 
and  when  pressure  is  exerted  on  the  keel,  the  stamens  and  pistil 
spring  out  of  their  enclosure  with  considerable  force.     {Fig.  22.) 


B  ^ 

Fig.  23.  —  Flowers  of  the  Yarrow  (Achillea 
millefolium),  a  Composite.  A,  a  head  of 
flowers  sectioned,  showing  the  strap-shaped 
flowers  around  the  margin  and  the  tubular 
flowers  occupying  the  central  region  of  the 
head.  B  and  C  are  tubular  and  strap- 
shaped  flowers  more  enlarged 


A 

Fig.  24.  —  A,  flower  from 
the  head  of  Dandelion,  a, 
strap-shaped  corolla;  b,  calyx 
made  up  of  many  slender  hairs 
known  as  pappus;  p,  base  of 
pistil;  .s,  stamens  forming  a 
tube  around  the  upper  part  of 
'  the  pistil.  B,  tubular  flower 
and  fruit  of  Beggar's  Tick 
showing  tubular  corolla  (a)  and 
the  calyx  (b)  consisting  of  two 
spiny  teeth  which  persist  and 
aid  in  scattering  the  fruit. 


This  process  of  releasing  the  stamens  and  pistil,  known  as 
"  tripping  the  flower,"  is  mainly  the  work  of  insects  and  is  im- 
portant, because  in  some  of  the  Legumes  the  flowers  will  produce 
no  seed  unless  tripped. 

Composite  Flowers.  —  There  is  a  large  group  of  plants  to 
which  Lettuce,  Dandelions,  Sunflowers,  Beggar's  Tick,  Thistles, 


COMPOSITE   FLOWERS 


25 


and  many  other  plants  belong,  that  have  their  many  small  flowers 
grouped  in  a  compact  head  as  shown  at  A  in  Figure  23.  This 
group  of  plants  is  called  Composites,  and  includes  some  of  our 
useful  plants  as  well  as  some  of  the  most  troublesome  weeds. 


Fig.  25.  —  A  cluster  of  Lady's-slippers. 

Both  calyx  and  corolla  are  somewhat  peculiar.  In  some  cases, 
as  in  the  Sunflower,  the  flowers  occupying  the  center  of  the  head 
have  tube-hke  corollas  and  are  called  tubular  flowers,  while  those 
around  the  margin  have  strap-shaped  and  much  more  showy 
corollas,  and  are  called  ligulate  flowers.  See  A,  B,  and  C  of 
Figure  23.  In  some  of  the  Composites,  as  in  the  Dandelion,  all 
of  the  flowers  of  the  head  are  ligulate,  while  in  some,  like  the 
Thistle,  all  the  flowers  are  tubular.     The  calyx  is  often  composed 


26 


FLOWERS 


of  hair-like  structures  called  pappus,  as  shown  in  Figure  24-  In 
some,  as  the  Dandelion  illustrates,  the  pappus  remains  after  the 
seed  is  mature,  forming  a  parachute-like  arrangement  which 
assists  in  floating  the  seed  about.  In  some  of  the  Composites, 
the  calyx  consists  of  a  few  teeth,  which  in  the  Spanish  Needles 
and  Beggar's  Tick,  become  spiny,  and 
thcreb}^  assist  in  seed  distribution  by 
catching  onto  passing  objects. 

Orchid  Flowers.  —  It  is  among 
Orchid  flowers,  many  of  which  are 
spectacular,  that  the  most  notable 
irregularities  occur.  Besides  the  dis- 
tinguishing feature  of  having  the 
stamens  and  pistil  joined  into  one 
body,  known  as  the  column,  Orchid 
flowers  often  have  pronounced  varia- 
tions in  the  shape  and  size  of  petals. 
In  some,  as  in  the  Lady's-slipper,  one 
of  the  petals  is  developed  into  a  great 
sac  or  "  slipper,"  while  the  others 
have  no  extraordinary  features.  These 
peculiarities  in  flower  structure,  which 
are  apparently  adjustments  for  insect 
pollination,  sometimes  so  closely  con- 
form to  the  shape  and  habit  of  cer- 
spadix  which  is  enclosed  in  the  tain  insects  that  only  one  or  a  few 
large  leaf-hke  bract  or  spathc.  kinds  of  insects  can  pollinate  a  flower. 
Reduced  about  one-half.  g^^^  highly  modified  flowers  contrast 

strikingly  with  the  simple,  inconspicuous  flowers  of  such  plants 
as  the  Jack-in-the-pulpit  or  Indian  Turnip  and  Skunk  Cabbage, 
in  which  a  perianth  is  either  lacking  or  inconspicuous  and  the 
flowers  are  crowded  on  a  fleshy  spike,  known  as  a  spadix,  which 
is  enclosed  in  or  attended  by  a  leaf,  called  spathe.  The  spathe, 
by  becoming  colored,  often  aids  like  a  corolla  in  attracting 
insects.     (Figs.  25  and  26.) 


Fig.  26.  —  The  inconspicu- 
ous flowers  of  the  Indian 
Turnip  (Arisoema  Iriphyllum) . 
The  flowers  shown  are  pistil- 
late and  are  clustered  at  the 
base    of    the    fleshy 


Arrangement  of  Flowers  or  Inflorescence 

The  arrangement  of  flowers  on  the  stem  is  one  of  the  floral 
characters  much  used  in  the  classification  of  the  Flowering  Plants. 
In  the  arrangement  of  flowers,  a  number  of  things  are  considered. 


ARRANGEMENT  OF   FLOWERS  OR   INFLORESCENCE       27 


the  principal  ones  being:  (1)  tlie  position  of  the  flower  on  the 
stem,  whether  terminal  or  lateral;  (2)  whether  the  flowers  are 
single  or  in  clusters;  (3)  whether  the  terminal  or  lateral  flowers 
of  a  cluster  open  first;  and  (4) 
the  character  of  the  cluster  in 
regard  to  shape  and  compact- 
ness, which  depend  upon  the 
elongation  of  the  stem  region 
bearing  the  flowers  and  the  length 
of  the  individual  flower  stalks. 
These  features  taken  singly,  to- 
gether, and  along  with  some 
minor  features  form  the  basis 
upon  which  floral  arrangements 
are  classified. 

Flowers  develop  from  buds 
and  buds  are  either  terminal  or 
lateral  on  the  stem.  So  as  to 
position,  flowers  are  either  ter- 
minal or  lateral  on  the  flower  axis. 
Flowers  borne  singly  are  called 
solitary  flowers,  and  solitary  flowers  may  be  terminal,  as  in  some 


.27.  —  Solitary  terminal  flower 
of  a  Lily.     After  Andrews. 


Fig.  28.  —  A  portion  of  a  Squash  plant  showing  the  axillary  arrangement 
of  flowers.     IVIuch  reduced. 

Lilies  of  which  the  Tulip  is  an  example,  or  lateral,  as  Squashes 
illustrate.     {Figs.  27  and  28.) 

The  flower  cluster  may  be  regarded  as  a  modification  of  that 
lateral  arrangement,  in  which  the  flowers  are  scattered  on  a  fully 


28 


FLOWERS 


elongated  stem  bearing  normally  developed  leaves  in  the  axils 
of  which  the  flowers  occur.  Thus,  if  a  Pumpkin  or  Gourd  vine 
should  remain  short,  the  flowers  instead  of  being  well  separated 
as  they  normally  are,  would  be  crowded,  and,  with  the  reduction 
of  leaves  to  bracts,  a  typical  flower  cluster  would  result.  Most 
small  flowers  are  produced  in  clusters.  For  small  flowers  polli- 
nated by  insects,  there  is  considerable  advantage  in  the  cluster 
habit,  since  the  cluster,  being  much 
more  conspicuous  than  the  individual 
flowers,  serves  well  as  an  attractive 
device. 

Flower  clusters  are  divided  into 
two  main  classes  according  to  their 
method  of  development.  In  the 
corymbose  or  indeterminate  cluster, 
growth  at  the  tip  and  the  develop- 
ment of  new  flowers  just  behind 
continues  throughout  a  considerable 
period,  thus  producing  a  cluster  in 
which  the  older  flowers  are  left 
farther  and  farther  behind.  As  the 
term  indeterminate  suggests,  such  a 
method  of  development  permits  a 
rather  indefinite  expansion  of  the 
cluster.  In  the  cymose  or  determi- 
nate cluster,  the  oldest  flower  is 
formed  at  the  tip,  which  is  thereby 
closed  to  further  growth,  and  the 
new  flowers  are  formed  from  buds 
developing  lower  down.  Such  a 
cluster  is  much  limited  in  its  power 
to  expand.  The  flower  clusters  of  Apples  and  Pears,  known  as 
cymes,  illustrate  the  determinate  type  of  cluster. 

The  simplest  form  of  the  indeterminate  cluster  is  the  raceme, 
an  unbranched  cluster  in  which  the  flowers  are  borne  on  short 
stalks.  The  racemes  of  the  Shepherd's-purse,  Radish,  Cabbage, 
and  others  of  the  Mustard  family,  in  which  the  flower  cluster 
may  continue  its  expansion  for  a  long  period,  producing  new 
flowers  at  the  tip  while  pods  are  maturing  at  the  base,  well 
illustrate  the  nature  of  the  raceme.     {Fig.  29.)     The  racemes  of 


Fig.  29.  —  Raceme  of  Com- 
mon Cabbage  {Brassica) .  From 
Warming. 


ARRANGEMENT  OF  FLOWERS  OR  INFLORESCENCE       29 

the  Snap-dragon,  Sweet  Clover,  and  Alfalfa  are  examples  of 
racemes  with  a  short  growth  period.  Racemes  may  be  terminal 
or  lateral,  as  in  case  of  Sweet  Clover. 


Fig.  30.  —  A,  spike  of  Rye.     B,  panicle  of  Grass.     C,  flowers  of  the  Hazel 
with  staminate  flowers  in  catkins  and  the  pistillate  flowers  borne  singly. 


Fig.  31.  —  A,  head  of  Clover.     B,  close  head  of  Yellow  Daisy. 

Raceme-like  clusters  in  which  the  flowers  have  very  snort 
stalks  or  none  at  all  are  called  spikes  of  which  the  heads  of  Wheat 
and  Timothy  are  familiar  examples.     A  special  form  of  the  spike 


30  FLOWERS 

is  the  catkin  in  which  the  flowers,  unisexual  in  t3'pical  cases, 
usually  have  scaly  bracts  instead  of  a  true  perianth,  and  the 
whole  cluster  falls  after  fruiting.  Catkins  are  typical  of  Poplars, 
Willows,  Hickories,  and  Birches.  When  the  raceme  is  so  short 
that  the  compact  mass  of  flowers  form  a  more  or  less  rounded 
cluster  as  in  Red  Clover,  then  a  head  is  formed.  In  the  Composites 
there  is  the  special  kind  of  head  which  is  the  most  highly  organ- 
ized of  all  flower  clusters.  The  flowers  besides  often  being  differ- 
entiated into  two  kinds  are  so  compactly  arranged  as  to  form  a 
cluster  resembling  a  single  flower  and  the  cluster  is  surrounded  by 
bracts,  which  form  a  stnacture  known  as  the  involucre.     {Fig.  31.) 


B 

Fig.  32.  —  A,  Corymb  of  one  of  the  Cherries,     B,  umbel  of  a  species 
of  Onion. 

In  contrast  to  the  spike  there  are  those  raceme-like  clusters 
in  which  the  flowers  have  long  stalks,  as  in  the  typical  panicle, 
where  the  cluster  is  loosely  branched.  When  the  portion  of  stem 
to  which  the  flowers  are  attached  is  short  and  the  stalks  of  all  of 
the  flowers  are  so  elongated  as  to  bring  all  of  the  flowers  to  about 
the  same  level  then  a  corymb  results.  A  further  modification  in 
which  the  portion  of  stem  to  which  the  flowers  are  attached  is  so 
short  that  the  flower  stalks  appear  to  be  of  the  same  length  and 
attached  in  a  circle  around  the  stem  results  in  the  umbel,  the  form 
of  cluster  characteristic  of  the  Parsley  Family,  called  Umbellif- 
ercB,  on  account  of  the  character  of  the  flower  cluster.  Of  this 
family  the  Parsnips,  Carrots,  and  others  are  common.  The  um- 
bel is  also  common  among  the  Milkweeds.  Umbels  may  be 
simple  or  compound,  that  is,  so  branched  as  to  be  composed  of 
a  number  of  small  umbels.     (Fig.  32.) 


ARRANGEMENT  OF   FLOWERS   OR  INFLORESCENCE       31 


Fig.  33.  —  A,  cyme  of  the  Apple.     B,  thyrsc  of  the  Lihic. 


32 


FLOWERS 


In  complex  flower  clusters  coinbinations  of  the  simpler  types 
of  clusters  often  occur  together.  Thus,  in  the  thyrse,  the  complex 
cluster  which  is  typical  of  the  Lilac  and  Horse-Chestnut,  and,  in 


Fig.  34.  —  Upper  diagrams  show  types  of  indeterminate  inflorescences. 
a,  raceme;  b,  corymb;  c,  compomid  corymb;  d,  mnbel;  e,  spike;  /,  panicle; 
g,  head. 

Lower  diagrams  show  types  of  determinate  inflorescences;  h,  cyme  half 
developed  (scorpioid);  i,  flat-topped  or  corymbose  cyme;  j,  typical  cyme. 


the  panicle  of  the  Grasses,  the  characteristics  of  both  racemes  and 
cymes  are  present.     (Fig.  33.) 

The  diagrams  in  Figure  34  show  the  common  types  of  flower 
arrangements. 


CHAPTER  IV      V 
PISTILS   AND    STAMENS 
Structure  and  Function  of  Pistils  and  Stamens 

The  pistils  and  stamens  are  the  organs  upon  which  the  pro- 
duction of  seed  depends  and  for  this  reason  are  called  the 
essential  parts  of  the  flower.  The  calyx  and  corolla  protect  the 
essential  organs  and  often  assist  in  seed  production,  but  they 
are  not  essential. 

In  unisexual  flowers,  seeds  appear  only  in  the  flowers  having 
pistils.  The  staminate  flowers  in  the  Corn  tassel  produce  no 
kernels,  and  in  dioecious  plants, 
such  as  Hemp,  Willows,  and  the 
Mulberry,  seed  and  fruit  are 
limited  to  those  individuals  bearing 
pistillate  flowers.  From  this  it 
might  appear  that  the  stamens 
take  no  part  in  the  work  of  pro- 
ducing the  seed;  but  observations 
show  that  unless  stamens  are  close 
at  hand,  the  pistil  will  produce 
no  seed.  A  well  isolated  Corn 
plant  with  tassel  removed  before  Fig,  35.  — Flower  of  the  Cherry 
the  stamens  are  mature  will  pro-  with  parts  of  the  pistil  indicated. 
J  1  1         CI  •  J.-        o,    ovary;    st,    stigma;    s,    style. 

duce  no   kernels.     Some  varieties  J.    ,      ,       •, 
„  „^         ,        .  ,.       .  ,    Much  enlarged. 

of  Strawberries  are  dioecious,  and 

unless  both  kinds  of  plants  are  grown  in  the  same  bed,  there  will 
be  no  seed  or  fruit. 

To  understand  just  how  the  essential  organs  function  in  seed 
production,  a  careful  study  of  their  parts  must  be  made. 

Parts  of  the  Pistil.  — ■  The  pistil  usually  consists  of  three  parts: 
the  enlarged  base  which  is  the  ovary  and  the  portion  in  which  the 
seeds  develop;  the  flattened  or  expanded  surface  at  the  upper 
extremity,  known  as  the  stigma;  and  the  stulk-like  part  connect- 

33 


34 


PISTILS  AND   STAMENS 


ing  the  ovary  and  stigma,  known  as  the  style.     In  the  pistil  of  the 

Cherry  shown  in  Figure  So  the  parts  are  indicated.     The  ovary  is 

at  0.     The  stigma  is  the  expanded  surface  at  st.     The  style  is  at  s 

and  is  a  stalk-like  structure  projecting  from 

the  ovary  and  supporting  the  stigma. 

In  the  Corn  the  style  is  extremely  long  and 
the  stigma  branched.  {Fig.  36.)  In  Wheat, 
Oats,  Barley,  and  Rice  there  are  two  very 
short  styles  and  the  stigmas  are  much 
l)ranched  and  plume-like.  {Fig.  37.)  Styles 
and  stigmas  vary  much  among  plants. 

Ovary.  —  The  ovary  is  the  most  impor- 
tant part  of  the  pistil  because  within  it  the 
seeds  are  produced,  and  often  it  makes  the 
edible  portion  of  fruits. 


Fig.  36.  —  Pistillate 
flower  of  Corn,  drawn 
to  show  the  parts  of 
the  pistil.  A  portion 
of  the  bracts  have 
been  cut  away  to  give 
a  view  of  the  ovary. 
0,  ovary,  the  portion 
that  becomes  the  ker- 
nel; s,  style;  s^,  stigma. 
Much  enlarged. 


Fig. 

Wheat 

lodicules 

stigmas; 


37.  —  Pistil  of 

and    the    two 

0,  ovary;  st, 

s,    styles;    /, 


lodicules.      Much   en- 
larged. 


Fig.  38.  —  Cross  section 
of  the  ovary  of  a  Tomato. 
0,  ovary  wall;  b,  partition 
walls  of  the  ovary ;  c,  locules 
or  cavities  in  the  ovary;  d, 
ovules;  p,  placentas  or  parts 
of  the  ovary  to  which  the 
ovules  are  attached.  Much 
enlarged. 


When  the  ovary  is  sectioned  so  that  its  interior  may  be  studied, 
it  is  seen  that  it  is  not  a  solid  body,  but  consists  of  a  wall 
enclosing  one  or  more  cavities,  called  locules.  {Fig.  38.)  In 
these  cavities  or  locules  are  the  small  bodies  called  ovules,  each 
of  which  is  capable  of  developing  into  a  seed.  Point  out  the 
parts  of  the  ovary  shown  in  Figure  38. 

The  ovary  may  contain  one  locule  or  many  and  the  number  of 
ovules  in  a  locule  also  varies  in  different  ovaries.    In  Beans  and 


OVARY 


35 


Fig.  39.  —  Flower  and  pod  of  the  Garden  Pea.  A,  section  through  the 
flower  to  show  ovules,  a,  ovary;  o,  ovules;  b,  stamens;  /,  stigma;  s,  style. 
B,  the  matured  ovary,  called  pod,  opened  to  show  the  matured  ovules  or 
seeds  (e).     Flower  enlarged  but  pod  less  than  natural  size. 


Fig.  40.  —  A,  pistil  of  Red  Clover  with  one  side  of  ovary  cut  nway  so  that 
the  ovules  (o)  may  be  seen,  a,  stigma;  .s,  style.  B,  lengthwise  section 
through  the  ovary  and  ovules  of  Red  Clover  and  very  nuicli  enlarged  to 
show  the  parts  of  the  young  ovules.  u\  ovary  wall;  o,  ovules;  s,  base  of 
style;   st,  stalk  or  funiculus  of  the  ovules;    /(,  nucellus;   /,  integuments. 


36 


PISTILS  AND   STAMENS 


Peas,  the  ovary  has  one  locule  enclosing  a  number  of  ovules.  In 
A  of  Figure  39,  showing  a  lengthwise  section  through  the  flower 
of  the  Pea,  one  side  of  the  ovar}^  wall  is  removed  to  show  the 
locule  with  its  ovules.  In  this  particular  flower  of  the  Pea,  there 
are  six  ovules,  but  other  flowers  might  have  more  or  fewer.  In 
B  of  Figure  39  is  shown  the  ovary  after  it  becomes  a  mature  pod. 
The  pod  is  opened  to  show  the  seeds.  Each 
seed  is  a  developed  ovule  and  the  pod  enclos- 
ing the  seeds  is  the  ovary  wall  much  enlarged. 
Notice  how  the  ovules  and  seeds  compare  in 
number. 

In  Red  Clover,  shown  in  Figure  40,  there  is 
one  locule  and  two  ovules.  The  ovaries  of 
Alfalfa  have  only  one  locule,  but  may  have  as 
many  as  eighteen  ovules. 

In  the  ovary  of  Corn,  Wheat,  Oats,  and 
Grasses  in  general,  there  is  one  locule  and  a 


Fig.  41.— Length, 
wise  section  througl 
a    young    pistil 


of 


Corn  to  show  the 
locule  and  o\ailc. 
a,  ovary;  s,  style; 
0,  ovule  consisting 
of  nucellus  (n)  and 
integuments  (i);  I, 
locule  or  cavity  in 
which  the  ovule  is 
located.  Much  en- 
larged. 


Fig.  42.  —  Lengthwise   section   through  a   Tomato 

flower  to  show  the  interior  of  the  ovary,  a,  ovary; 

I,    locules,    represented  by   dark  shading;  o,   ovules; 
p,  placentas.     Much  enlarged. 


single  large  ovule.  A  lengthwise  section  through  the  pistil  of 
Corn  is  shown  in  Figure  4^-  Notice  the  ovule  at  o  and  that  it 
almost  fills  the  locule. 

Tomato  ovaries  have  few  or  many  locules  which  contain  a  large 
number  of  ovules.  Figure  4^  shows  a  lengthwise  section  of  a 
Tomato  ovary  showing  two  locules  and  many  ovules.    By  count- 


SIZE   OF  OVULES  37 

ing  the  ovules  shown  in  Figure  4^  and  those  shown  in  Figure  38 
the  number  of  ovules  in  a  Tomato  may  be  roughly  estimated. 

An  examination  of  the  ovaries  of  many  plants  would  show 
considerable  variation  in  the  number  of  locules  and  ovules,  but 
in  general,  all  ovaries  consist  of  an  ovary  wall  enclosing  one  or 
more  locules  which  contain  one  or  more  ovules. 

Ovule.  —  Since  ovules  develop  into  seeds,  they  have  the  most 
to  do  with  seed  production  and  are,  therefore,  the  most  directly 
related  to  the  function  of  the  flower.  The  process  of  fertilization, 
one  of  the  most  important  events  in  plant  life,  takes  place  in  the 
ovule  and  a  good  understanding  of  fertilization  requires  a  knowl- 
edge of  the  ovule. 

Size  of  Ovules  and  how  their  Number  Compares  with  the  Num- 
ber of  Seeds.  —  Although  ovules  are  the  chief  structures  in  per- 
forming the  function  of  seed-production,  in  size  they  are  usually 
very  inconspicuous  and  not  much  can  be  learned 
about  them  without  the  aid  of  the  microscope. 
In  many  plants  the  ovules  are  barely  visible  to 
the  unaided  eye.  When  ovaries  and  ovules  are 
shown  in  drawings,  they  are  usually  much  en- 
larged, so  that  much  more  is  shown  than  could  .^l^'n.  "  7~,  f  ^ 
.  .  ,  1    •  ,       of  the  Tomato  taken 

be  seen  by  cuttmg  sections  and  studymg  the  from  the  flower  and 
ovaries  themselves,  unless  a  microscope  were  drawn  natural  size. 
used.  In  Figure  43,  the  pistil  of  the  Tomato  is 
shown  natural  size.  By  comparing  it  with  the  pistil  shown  in 
Figure  4^,  it  will  be  seen  that  in  order  to  show  the  structures  of 
the  ovary,  the  pistil  in  the  latter  Figure  is  much  enlarged. 

Since  ovules  are  small,  it  is  difficult  to  count  them  in  ovaries 
where  they  are  numerous.  It  is  possible  in  many  cases  to  make 
a  rough  estimate  of  the  number  of  ovules  by  counting  the  seeds 
produced.  Since  each  seed  is  a  developed  ovule,  there  must 
occur  in  the  young  ovary  as  many  ovules  as  there  are  seeds  in  the 
mature  ovary.  From  this  it  follows  that  those  Tomatoes  con- 
taining two  hundred  or  more  seeds  must  have  had  as  many  ovules 
in  their  young  ovaries. 

If  all  the  ovules  became  seeds  then  a  count  of  the  seeds  would 
give  the  exact  number  of  ovules;  but  in  many  cases,  due  to  a  lack 
of  fertilization,  space,  or  sufficient  food  supply,  only  a  part  of  the 
ovules  complete  their  development  and  become  seeds.  In  Red 
Clover,  as  shown  in  Figure  40,  there  are  two  ovules,  but  when  the 


38 


PISTILS  AND  STAMENS 


mature  pod  is  threshed,  only  one  seed  is  found.  In  Alfalfa  only 
about  one  third  of  the  ovules  produce  seed.  In  the  Apple,  Pear, 
Tomato,  and  other  fruits  some  of  the  ovules  often  fail  to  develop, 
and  in  case  of  seedless  fruits  none  of  the  ovules  complete  their 
development.  In  most  fruits  the  production  of  seed  is  not  an 
important  feature  to  the  plant  grower,  the  seedless  fruit  in  many 
cases  being  more  desirable;  but  in  case  of  Clover,  Alfalfa,  Flax, 
and  other  plants  valuable  for  seed,  the  value  of  the  plant  as  a  seed 
producer  is  directly  related  to  the  number  of  ovules  which  be- 


Fig.  44.  —  Surface  view  of  an  ovule 
at  two  stages  of  development.  ^1, 
stage  of  development  showing  the 
integuments  (o,  b)  growing  up  over 
the  nucellus  (n).  B,  older  stage  in 
which  the  integuments  have  closed 
over  the  nucelkis,  leaving  only  a 
small  opening,  the  micropyle  (m).  s, 
the  funiculus.     Much  enlarged. 


tn 

Fig.  45.  —  Section  through 
the  ovule  of  Red  Clover  show- 
ing the  embryo  sac.  em,  em- 
bryo sac  with  the  egg  (e)  and 
the  primary  endosperm  nucleus 
(en)  indicated;  i,  integuments; 
m,  micropyle.  Many  times 
enlarged. 


come  seed.  How  much  could  the  seed  yield  of  Clover  and  Alfalfa 
be  increased  if  they  could  be  made  to  develop  all  of  their  ovules 
into  seed?  If  clover  seed  were  selling  at  $10  per  bushel,  what 
would  be  the  value  of  the  increased  yield  on  ten  acres  of  average 
Clover? 

Parts  of  the  Ovule.  —  The  ovule  consists  of  a  main  body  and 
a  stalk  known  as  the  funiculus  which  connects  to  the  ovary  wall. 
The  main  body  consists  of  a  central  (usually  rounded)  portion 
called  nucellus,  which  is  enclosed  by  one  or  more  coverings  called 
integuments  that  grow  up  from  the  funiculus.  In  Figure  Ifi, 
showing  the  ovules  of  Clover,  the  stalk  or  funiculus  is  at  st;  the 
central  portion  or  nucellus  of  the  main  body  is  at  n;  the  coverings 
or  integuments  of  the  nucellus  are  at  i.  Turn  to  this  Figure  and 
point  out  these  parts.  In  the  ovule  of  the  Corn,  shown  in  Figure 
41,  the  funiculus  is  apparently  absent.     In  Figure  44  is  shown  a 


HOW  THE  PARTS  OF  AN  OVULE  ARE  MADE  UP 


39 


surface  view  of  an  ovule  at  two  stages  of  development.  Notice 
how  the  nucellus  is  enclosed  by  the  integuments,  leaving  only 
a  small  opening  at  m  known  as  the  micropyle. 

The  pollen  tube,  a  tube-like  structure  produced  by  the  pollen 
grain  in  connection  with  fertilization,  often  uses  the  micropyle  as 
an  entrance  to  the  ovule.  Some  ovules  are  straight  but  oftener 
there  is  a  curving  to  one  side  during  growth  as  shown  in  Figure  44- 
By  curving  the  micropyle  is  brought  near  the  base  of  the  ovule,  a 
position  more  favorable  for  the  entrance  of  the  pollen  tube. 

How  the  Parts  of  an  Ovule  are  made  up.  —  The  ovule,  like  all 
other  parts  of  the  plant,  is  made  up  of  many  living  units  called 
cells.  A  cell  consists  of  a  mass  of  _ 
Hving  matter  called  protoplasjn, 
which  is  generally  enclosed  by 
walls.  A  very  important  part 
of  the  living  matter  is  the  nu- 
cleus, a  globular  body  commonly 
occupying  a  .  central  position  in 
the  cell.  The  ovule,  although  a 
very  small  body,  is  composed  of 
many  hundreds  of  cells,  all  of 
which  are  in  some  way  related  to 
seed  formation. 

The  cells  of  the  funiculus,  in- 
teguments, and  most  of  those  of 
the  nucellus  furnish  food  and  de- 
velop a  covering  for  the  inner  and 
more  vital  parts  of  the  seed.  In 
form  and  structure  they  are 
similar  to  cells  composing  other 
parts  of  the  plant.  The  cells 
peculiar  to  the  ovule  are  those  forming  a  special  group,  usually 
seven  or  eight  in  number  and  occupying  a  central  position  in  the 
nucellus  One  peculiar  feature  of  these  cells  is  that  they  usually 
are  not  separated  by  cell  walls  and  their  masses  of  protoplasm  lie 
in  contact  or  closely  join  with  each  other.  The  region  which 
these  cells  occupy  is  known  as  the  embryo  sac,  so  named  because 
within  it  the  embryo  develops.  The  embryo  sac,  being  deeply 
buried  in  the  nucellus  wh  ch  is  in  turn  enclosed  by  the  integu- 
ments, is  well  protected  and  to  study  it  the  ovule  must  be  sec- 


FiG.  46.  —  Lengthwise  section 
through  the  ovary  of  Corn  showing 
embryo  sac.  o,  ovule;  em,  embryo 
sac;  e,  egg;  en,  the  two  nuclei  which 
fuse  to  form  the  primary  endosperm 
nucleus;  i,  integuments;  w,  ovary 
wall;  s,  base  of  style  or  silk.  Much 
enlarged. 


40 


PISTILS   AND   STAMENS 


tioned.  In  some  ovules  the  embryo  sac  may  be  seen  without 
the  microscope,  but  in  most  ovules  it  is  microscopic.  There  is 
only  one  cell  and  one  nucleus  in  the  embryo  sac,  which  have  an 
important  function  in  the  formation  of  the  seed.  The  important 
cell  is  the  egg.  The  egg  is  at  the  micropylar  end  and  after 
fertilization  produces  the  embryo  of  the  seed.  The  important 
nucleus,  referred  to  as  nucleus  because  it  has  no  definite 
amount  of  protoplasm,  is  the  primary  endosperm  nucleus.  It 
is  near  the  center  of  the  embryo  sac  and  is  important  because 
upon  it  the  development  of  the  stored  food  or  endosperm  of 
the    seed    depends.      The   remaining   cells   and    nuclei   of    the 


Fig.  47.  —  A  vertical  section  through  an  Oat  ovary  to  show  the  parts  of 
the  ovule.  Parts  of  the  lemma,  palea,  and  two  stamens  are  shown,  and  one 
style  and  stigma  remains.     Label  the  parts  of  the  ovule.     Much  enlarged. 


embryo  sac  are  absorbed  and  disappear  soon  after  the  egg  is 
fertilized.  In  the  ovules  of  Clover  and  many  other  plants,  the 
cells  at  the  inner  end  (chalazal  end)  of  the  embryo  sac  disappear 
even  before  the  egg  is  fertilized. 

A  section  through  an  ovule  of  Red  Clover  is  shown  in  Figure 
45.  Point  out  the  embryo  sac.  Notice  the  egg  at  e  and  the 
endosperm  nucleus  at  en.  Point  out  the  embryo  sac  of  Corn  in 
Figure  1^6.  Notice  that  instead  of  a  single  primary  endosperm 
nucleus,  there  are  two  nuclei  lying  in  contact.  These  nuclei  fuse 
and  form  the  primary  endosperm  nucleus.  A  section  through 
an  ovule  of  Oats  is  shown   n  Figure  1^1.     Point  out  the  embryo 


^ 


THE   POLLEN   GRAIN   AND    ITS   WORK  41 

sac,  egg,  and  primary  endosperm  nucleus.     Redraw  this  figure 
on  a  sheet  of  paper  and  label  the  parts. 

Although  pistils  vary  much  in  number  of  carpels,  length  of 
styles,  and  in  number  of  locules  and  ovules,  there  is  uniformity 
in  organization  and  adaptation  of  parts  to  special  functions. 
The  stigma  is  especially  adapted  for  receiving  pollen,  the  style 
supports  the  stigma  in  a  position  suitable  for  receiving  the  pollen, 
and  the  ovary  protects  the  delicate  ovules  in  which  is  the  embryo 
sac  containing  the  egg  and  primary 
endosperm  nucleus,  which  are  the 
chief  structures  of  the  pistil. 

The  Stamen.  —  The  stamen  usu- 
ally consists  of  two  parts;  the  en- 
larged terminal  portion,  or  anther; 
and  the  stalk,  or  filament.  The 
filament  is  often  so  short  as  to  seem 
to  be  absent.     Point  out  the  parts 

of  the  stamen  in  A  of  Figure  48. 

rrir  j.u       •  n     r         i   i,     i        FiG.  48.  —  A,  Stamen,     a,  an- 

The  anther  is  usually  four  lobed   .u       f  a^        \      t>         u 

.  Ill-  •  ther;   /,  filament.     B,  much  en- 

and  Wlthm  each  lobe  is  a  cavity,  larged  cross  section  of  an  anther, 
called  locule,  which  contains  many  showing  the  locules  and  pollen 
globular  bodies  known  as  pollen  or  grains.  The  two  locules  at  the 
pollen  grains.  When  the  pollen  is  ^''^{igj^''7gg°^'J|;'''^'  ^"^^^^^  ^^'^ 
mature,  the  walls  of  the .  anther  ^°  ^"  escape, 
open  and  allow  the  pollen  to  escape.  Notice  the  cross  sec- 
tion of  an  anther  shown  in  B  of  Figure  J/S-  Point  out  the 
locules  and  pollen  grains.  Notice  that  two  of  the  locules  have 
opened. 

The  Pollen  Grain  and  its  Work.  —  The  pollen  grain  is  a  cell 
with  its  living  matter  enclosed  in  a  heavy  protective  wall.  It 
needs  to  be  well  protected,  for  during  its  journey  to  the  pistil, 
destructive  agencies  such  as  cold,  heat,  and  drying  are  encoun- 
tered. The  transference  of  the  pollen  to  the  stigma  is  called 
pollination.  Pollination  is  a  very  important  event,  for  the  pollen 
cannot  perform  its  function  except  on  the  stigma. 

On  the  stigma  the  pollen  grain  grows  a  tube  which  traverses 
the  stigma  and  style,  pierces  the  ovule,  and  reaches  the  embryo 
sac.  Pollen  grains,  when  first  formed  in  the  anther,  have  only 
one  nucleus,  but  in  preparation  for  the  work  of  fertilization,  th(M-e 
is  nuclear  division  and  as  a  result  there  are  three  nuclei  in  a  well 


42 


PISTILS  AND  STAMENS 


developed  pollen  tube.  This  feature  is  shown  in  Figure  49.  The 
nucleus  at  the  end  of  the  tube  and  known  as  tube  nucleus  directs 
the  growth  of  the  tube  and  disappears 
soon  after  reaching  the  embryo  sac. 
The  two  nuclei  following  closely  be- 
hind the  tube  nucleus  are  the  sperms 
or  male  nuclei,  the  structures  which 
join  with  the  egg  and  primary  endo- 
sperm nucleus  in  fertilization.  The 
pollen  tube  is  a  passage  way  through 
which  the  sperms  pass  to  the  embryo 
sac. 

Fertilization. — After  the  two  sperms 
reach  the  embryo  sac,  one  approaches 
the  egg  and  fuses  with  its  nucleus,  while 


Fig.  49.  —  Pollen  grains  in  different  stages 
preparatory  to  fertilization.  A,  surface  view 
of  a  pollen  grain;  B,  section  through  pollen 
grain  in  uni-nucleate  stage;  C,  section  through 
pollen  grain  showing  the  nucleus  divided  into 
the  generative  (g)  and  tube  nucleus  (t);  D, 
pollen  tube  forming  into  which  the  two  nuclei 
have  passed;  E,  tube  more  developed  and 
generative  nucleus  divided  into  two  sperms 
(g).     Much  enlarged. 


Fig.  50.  —  A  diagram  of  a  length- 
wise section  through  the  pistil  of  Red 
Clover,  showing  pollen  tubes  trav- 
ersing the  stigma  and  style.  Two 
pollen  tubes  have  reached  the  em- 
bryo sacs,  p,  pollen  grains  develop- 
ing tubes;  st,  stigma;  p.t,  pollen 
tubes;  o,  ovules;  e,  egg;  en,  en- 
dosperm nucleus;  s,  sperms.  Much 
enlarged. 


the  other  approaches  the  primary  endosperm  nucleus  and  fuses  with 
it.    This  process  of  fusion  is  called  fertilization.    Since  there  are  two 


THE  DEVELOPMENT  OF  THE  OVULE  INTO  A  SEED   43 


fusions,  there  are  two  fertilizations,  and  the  two  fertihzations 
are  called  "  double  fertihzation."  Both  egg  and  primary  endo- 
sperm nucleus  are  now  said  to  be  fertilized,  and  the  pollen  grain 
has  performed  its  function,  which  is  an  important  one,  for  with- 
out fertilization  the  ovule  would  not  develop  into  a  seed. 
Pollination,  the  growth  of  the  pollen  tube  to  the  embryo  sac, 
and  the  formation  of  the  two  sperms  are  simply  preliminary 
acts  to  fertilization,  which  is  the  final  achievement  of  the  pollen 

grain.  Study  the  pollen  grains 
shown  in  Figure  49.  Notice  that 
the  tube  has  broken  through  the 


Fig.  5L  —  Stigma  of  Corn  show- 
ing how  the  pollen  grains  grow 
their  tubes  into  the  stigma,  p, 
pollen  grains;  f,  pollen  tube.  Much 
enlarged. 


Fig.  52.  —  A,  diagrammatic  section 
of  an  ovule  of  the  Tomato  in  which 
the  egg  (b)  and  primary  endosperm 
nucleus  (d)  have  been  fertilized,  o, 
portion  of  ovule  surrounding'  and  en- 
closing the  embryo  sac.  B,  diagram- 
matic section  of  the  seed  of  the 
Tomato,  e,  embryo;  c,  endosperm; 
t,  seed  coat.  The  lines  dra\vn  from 
the  ovule  to  the  seed  indicate  the 
parts  of  the  ovule  from  which  the 
different  parts  of  the  seed  have  de- 
veloped. Both  are  enlarged  but  the 
ovule  is  enlarged  much  more  than  the 
seed. 


pollen  wall.  How  have  the  two  sperms  been  formed?  In  Figure 
50  trace  the  pollen  tubes  to  the  embryo  sac.  How  do  the  pollen 
tubes  make  their  way  through  the  style?  Where  do  they  obtain 
their  food  for  growth?  Notice  how  the  pollen  tubes  enter  the 
liranched  stigma  of  Corn  in  Figure  51. 

The  Development  of  the  Ovule  into  a  Seed.  —  After  the  egg  and 
primary  endosperm  nucleus  have  been  fertilized,  the  ovule  begins 
its  development,  which  results  in  the  production  of  a  seed.     There 


44 


PISTILS  AND   STAMENS 


are  three  main  structures  involved  in  this  development:  (1)  the 
fertihzed  egg;  (2)  the  fertilized  primary  endosperm  nucleus; 
and  (3)  the  parts  of  the  ovule  surrounding  the  embryo  sac. 
The  development  of  each  of  these  parts  into  their  respective  seed 
parts  takes  place  simultaneously.  The  fertilized  egg  becomes 
the  embryo,  the  endosperm  nucleus  has  to  do  with  the  forming 
of  the  endosperm,  and  a  part  of  the  surrounding  portion  of  the 
ovule  becomes  the  seed  coat.     Figure  52  shows  a  Tomato  ovule 


Fig.  53.  —  A  young  ovary  of  Corn  just  after  fertilization  and  a  mature 
ovary  or  kernel,  both  of  which  are  sectioned  lengthwise  and  the  relation  of 
parts  indicated.  .4,  lengthwise  section  of  the  young  ovary  showing  nucellus 
(n),  egg  (e),  endosperm  nucleus  {en),  integuments  {i),  ovary  wall  {w),  and 
base  of  style  (fe).  B,  the  lengthwise  section  through  the  kernel  showing  the 
embryo  {em),  endosperm  {end),  seed  coat  (c),  ovary  wall  (w),  and  the  base 
of  the  style  {h).  The  dotted  lines  indicate  the  parts  of  the  ovule  from  which 
the  different  parts  of  the  kernel  have  developed. 


in  which  the  egg  and  endosperm  nucleus  have  just  been  fertilized 
and  also  shows  the  seed  which  develops  from  the  ovule.  The  lines 
indicate  the  parts  of  the  ovule  from  which  the  different  parts  of 
the  seed  have  come.  Study  Figure  53  showing  the  development 
of  the  ovule  of  Corn  into  a  seed.  Point  out  the  different  parts  of 
the  kernel  and  the  part  of  the  ovule  from  which  they  came. 
Notice  that  the  heavy  outer  covering  of  the  kernel  is  the  ovary 
wall,  and  does  not  come  from  the  ovule.  A  kernel  of  Corn  is  a 
seed  closely  jacketed  by  the  ovary  wall.     Copy  on  a  sheet  of 


THE  DEVELOPMENT  OF  THE  OVULE  INTO  A  SEED   45 


Fig.  54.  —  A,  a  vertical  section  through  an  Oat  ovary  showing  one  style 
and  stigma,  the  ovary  wall,  and  the  parts  of  the  ovule.  B,  a  vertical  section 
through  an  Oat  kernel  showing  its  parts.  After  comparing  with  Figure  53 
label  the  parts  of  A  and  B  and  with  lines  indicate  the  parts  of  A  from  which 
the  parts  of  B  have  developed. 


Fig.  55.  —  A  diagram  showing  the  relation  of  the  parts  of  the  ovule  to 
those  of  the  seed  in  Red  Clover.  A,  ovule  just  after  fertilization  showing 
the  egg  (e)  and  the  endosperm  nucleus  {d).  B,  seed  with  half  of  the  seed 
coat  (s)  removed  to  show  the  large  embryo  (em).  The  dotted  lines  indicate 
the  relation  of  the  parts  of  the  ovule  to  those  of  the  seed. 


46  PISTILS   AND   STAMENS 

paper  the  drawings  in  Figure  54  and  with  hnes  indicate  the  parts 
in  A  from  which  the  different  parts  shown  in  B  have  come. 

In  many  plants  the  endosperm  does  not  remain  outside  of  the 
embryo  as  it  does  in  Corn  and  other  grains.  If  one  removes  the 
thin  rind-Hke  testa  from  a  soaked  Bean,  all  that  remains  is  the 
large  embryo.  The  endosperm  is  stored  in  the  embryo  and  as  a 
result  the  embryo  is  much  enlarged  and  fills  the  space  within  the 
testa.  Clover,  Alfalfa  seed,  and  many  other  seeds  have  the  endo- 
sperm stored  in  the  embryo.  Study  the  Clover  seed  in  Figure  55. 
Notice  that  there  is  apparently  no  endosperm,  and  that  the  much 
enlarged  embryo  occupies  nearly  all  the  space  within  the  testa. 

In  some  seeds  a  stored  food  known  as  perisperm  occurs. 
Usually  as  the  ovule  develops  into  the  seed,  the  nucellus  is  de- 
stroyed and  replaced  by  the  developing  endosperm,  leaving  only 
the  integuments  from  which  the  seed  coat  is  formed.  However, 
in  the  formation  of  a  few  seeds,  some  of  the  nucellus  remains,  and 
a  portion  of  its  outer  region  becomes  filled  with  stored  food,  thus 
forming  the  layer  of  stored  food  known  as  perisperm,  which  sur- 
rounds the  endosperm  and  embryo. 

Pollinatioii 

Nature  of  Pollination.  —  Pollination  is  the  transference  of 
pollen  to  the  stigma.  After  the  pollen  is  on  the  stigma,  it  may 
produce  a  tube  reaching  to  an  ovule  and  effect  fertilization,  or 
it  may  lie  dormant;  but  in  either  case  the  stigma  is  considered 
pollinated.  Much  pollination  occurs  in  nature  that  does  not 
result  in  fertilization.  Corn  pollen,  for  example,  as  it  is  blown 
about  may  fall  on  the  stigmas  of  various  other  species  of  plants, 
but  since  no  fertilization  results,  the  pollination  is  not  effective. 
Pollen  is  usually  effective  only  on  stigmas  of  plants  similar  to 
the  plant  which  produced  the  poDen.  Thus  Apple  pollen  is 
effective  only  on  Apple  stigmas.  Corn  pollen  only  on  Corn 
stigmas,  etc. 

Pollinating  Agents.  —  The  most  important  pollinating  agents 
are  gravity,  wind,  insects,  and  man.  In  some  cases,  as  in  Rice, 
Wheat,  and  Oats,  where  the  pollen  falls  from  the  anthers  to  the 
stigma,  pollination  depends  upon  gravity.  Even  in  orchards 
some  pollination  may  be  accomplished  by  pollen  falling  from  the 
higher  branches.  ^  In  early  spring,  before  there  are  many  insects, 
many  of  our  trees,  such  as  Willows,  Poplars,  Oaks,  and  Pines, 


KINDS  OF   POLLINATION 


47 


depend  upon  the  wind  for  pollination.  The  wind  is  also  an 
important  agent  in  the  pollination  of  Corn  and  aids  some  in 
orchard  polhnation.  Plants  having  showy  flowers  depend  upon 
insects  for  pollination  and  it  is  among  these  plants  that  attractive 
colors,  secretions  of  nectar,  and  various  structural  arrangements, 
which  are  interpreted  as  adaptations  to  secure  pollination,  occur. 
The  pollination  of  Fruit  trees.  Clovers,  and  Alfalfa  is  done  chiefly 
by  insects.     {Fig.  56.)     In  experimental  work,  such  as  crossing 


Fig.  56.  —  Bumble  bee  pollinating  Red  Clover. 

Tomatoes,  Corn,  and  Fruit  trees,  man  himself  often  does  the 
pollinating  so  as  to  have  it  under  control. 

Kinds  of  Pollination.  —  On  the  basis  of  the  relation  of  the 
stamen  furnishing  the  pollen  to  the  pistil  pollinated,  there  can 
be  different  kinds  of  pollination.  The  transfer  of  pollen  from 
the  stamen  to  the  pistil  of  the  same  flower  is  self-pollination, 
while  the  transfer  to  the  pistil  of  another  flower  is  cross-pollina- 
tion.    Various  relationships  may  occur  in  polhnation.     Thus  the 


48  PISTILS  AND   STAiMENS 

pistil  of  a  Ben  Davis  Apple  blossom  may  be  pollinated:  (1)  with 
pollen  from  the  same  flower;  (2)  with  pollen  from  another  flower 
in  the  same  cluster;  (3)  with  pollen  from  a  flower  on  another 
branch;  (4)  with  pollen  from  another  Ben  Davis  tree  located  in 
the  same  or  a  neighboring  orchard;  or  (5)  with  pollen  from  a 
Jonathan  or  some  other  different  variety.  In  case  of  fruit  trees 
horticulturists  sometimes  consider  the  pistil  of  a  blossom  self- 
pollinated  if  the  pollen  comes  from  the  same  flower,  from  another 
flower  on  the  same  tree,  or  from  another  tree  of  the  same  kind, 
and  consider  the  pistil  cross-pollinated  only  when  the  pollen 
comes  from  another  variety  of  fruit  tree.  Corn  breeders  speak 
of  self-,  close-,  and  cross-pollination.  Pollination  resulting  from 
the  pollen  falling  from  the  tassel  to  the  silks  of  the  same  plant  is 
called  self-pollination.  Pollination  in  which  the  pollen  from  one 
plant  falls  on  the  silks  of  another  plant  is  called  close-pollination 
if  both  of  these  plants  came  from  kernels  taken  from  the  same 
ear,  but  cross-pollination  if  these  plants  came  from  kernels  taken 
from  different  ears.  In  case  of  cross-pollination,  the  plants  may 
be  of  the  same  variety  or  of  difTerent  varieties. 

The  Amount  of  Pollen  Required  for  Good  Pollination.  —  One 
pollen  grain  is  required  to  fertilize  each  ovule,  and,  therefore,  a 
pistil  with  many  ovules  requires  many  pollen  grains  for  good 
pollination.  In  Corn,  Wheat,  and  Oats  where  there  is  only  one 
ovule,  one  good  pollen  grain  on  the  stigma  is  sufficient,  although  a 
large  number  is  usually  present.  Due  to  the  great  waste  of  pol- 
len during  transportation,  much  more  is  produced  than  is  really 
needed.  A  medium-sized  plant  of  Indian  Corn  produces  about 
50,000,000  pollen  grains  or  about  7000  for  each  silk.  Many  of 
these  never  reach  a  silk,  and  of  the  many  that  do  all,  except  the 
one  that  reaches  the  ovule  first  with  its  tube,  accomplish  nothing. 
On  the  stigma  of  the  Red  Clover,  although  each  pistil  has  only 
two  ovules,  there  are  often  as  many  as  25  pollen  grains,  23  of 
which  are  wasted. 

On  the  other  hand,  in  flowers  where  the  ovaries  contain  numer- 
ous ovules,  as  in  Tomatoes  and  Melons,  it  often  happens  that 
not  enough  pollen  reaches  the  stigma  to  effect  fertilization  in  all 
the  ovules.  In  the  Tomato,  for  example,  an  ovary  may  contain 
as  many  as  200  ovules,  in  some  of  which  fertilization  may  not 
occur  because  of  insufficient  polhnation.  Even  in  Beans,  Apples, 
and  Pears,  where  the  ovules  are  not  numerous,  one  often  finds  in 


HOW   POLLEN   IS  AFFECTED    in'   EXTERNAL   FACTORS     49 

the  mature  fruit  some  undeveloped  ovules,  which  due  to  the  lack 
of  fertilization  did  not  become  seeds.  Although  nuich  of  the  vari- 
ation that  occurs  in  the  number  of  seeds  in  many  of  the  fruits  is 
due  to  the  failure  of  the  pollen  to  function  properly  on  the  stigma 
or  to  the  insufficient  nourishment  of  the  ovules,  nmch  of  the  vari- 
ation can  be  attributed  to  insufficient  pollination. 

There  is  good  evidence  that  the  imperfect  development  of 
fruit  is  due  in  some  cases  to  insufficient  polhnation.  By  polli- 
nating the  stigmas  of  Tomatoes  in  such  a  way  that  portions  of 
the  stigmas  received  no  pollen,  one  ^  investigator  found  that  no 
fertihzation  occurred  in  some  locules,  and  that  the  portion  of  the 
ovary  surrounding  these  locules  developed  much  less  than  those 
portions  of  the  ovary  surrounding  those  locules  in  which  fertili- 
zation occurred,  thus  causing  one-sided  fruits. 

How  Pollen  is  Affected  by  External  Factors.  —  Pollen  is  not 
so  specially  prepared  as  seeds  are  to  endure  extreme  conditions 
during  transportation.  During  transportation  and  while  on  the 
stigma,  pollen  may  be  either  killed  or  rendered  functionless 
by  extremes  of  temperature  and  moisture.  The  pollen  of  most 
plants  is  so  sensitive  to  dryness  that  an  exposure  to  the  ordinary 
dryness  of  the  air  cannot  be  endured  more  than  a  few  days  and 
in  many  cases  only  a  few  hours. 

In  the  storage  of  pollen,  which  is  sometimes  necessary  in  experi- 
mental work,  the  main  caution  is  to  store  the  pollen  where  it 
will  not  be  dried  out  too  much  by  evaporation,  although  the  pol- 
len must  be  kept  dry  enough  that  it  will  not  mold.  It  has  been 
found  that  Plum  and  Apple  pollen  can  be  kept  alive  much  longer 
when  stored  in  closed  chambers  where  there  is  less  drying  than 
in  laboratory  air.  One  investigator  has  reported  that  Corn  pol- 
len will  die  in  two  or  three  hours  when  exposed  to  the  air  of  the 
laboratory  or  living  room,  but  will  live  two  days  when  stored 
in  a  moist  chamber.  Some  investigators  think  that  hot  dry 
weather  during  the  pollination  of  fruit  trees  may  affect  the  setting 
of  fruit  by  destroying  some  of  the  pollen. 

The  pollen  of  some. plants,  as  in  case  of  Red  Clover  and  Alfalfa, 
absorbs  water  so  rapidly  that  it  is  destroyed  by  bursting  when 
immersed  in  water  or  stored  in  a  saturated  air.  Consequently 
these  plants  are  not  successfully  pollinated  when  they  are  wet 

1  Pollination  and  Rpproduction  of  Lycopersicum  esoulentum  (Tomato). 
Minnesota  Botanical  Studies,  p.  636,  Nov.  30,  1896. 


50  PISTILS  AND  STAMENS 

with  dew  or  rain.  Apple  pollen  and  the  pollen  of  many  other 
fruit  trees,  although  not  destroyed  when  immersed  in  water,  will 
not  function  nearly  so  well  and  for  this  reason  rain  or  dew  on 
a  stigma  may  hinder  the  pollen  in  its  work. 

The  pollen  of  manj'^  plants  is  quite  sensitive  to  a  low  tempera- 
ture, showing  a  decrease  in  vitality  when  exposed  for  a  few  hours 
to  a  temperature  only  a  little  below  freezing.  Pollen,  if  not  in- 
jured by  cold,  will  not  germinate  while  the  temperature  is  low. 
In  the  Apple,  Pear,  Plum,  Peach,  and  Cherry^  a  temperature  of 
—  1°C.  has  been  found  to  interfere  with  the  proper  functioning 
of  the  pollen  by  injuring  the  stigmas  and  preventing  the  ger- 
mination of  the  pollen.  Cold  during  the  blooming  period  may 
be  responsible  for  much  failure  in  fruit-setting. 

The  Results  of  Pollination.  —  The  most  immediate  as  well  as 
the  most  important  result  of  pollination  is  the  fertilization  of  the 
egg  cell  and  primary  endosperm  nucleus.  Through  the  process 
of  fertilization  the  pollen  stimulates  the  ovule  and  other  struc- 
tures to  develop,  and  transmits  factors  by  means  of  which  the 
embryo  and  the  endosperm  of  the  seed  inherit  the  characters  of 
the  pollen  parent. 

The  importance  of  the  stimulative  effect  of  fertilization  in 
the  development  of  a  seed  is  obvious,  for  unless  fertihzation 
occurs,  the  egg,  endosperm  nucleus,  and  other  parts  of  the  ovule 
rarely  develop  into  their  respective  seed  structures,  and  con- 
sequently the  ovule  either  disappears  or  remains  as  a  small 
withered  body  as  often  seen  in  fruits.  Furthermore,  the  devel- 
opment of  fruit  depends  upon  the  stimulative  effect  of  fertiliza- 
tion, as  shown  in  case  of  fruit  trees.  Melons,  Alfalfa,  etc.,  in  which 
the  flowers  wither  and  fall  from  the  plant  unless  fertilization 
occurs  in  some  of  the  ovules.  There  are,  however,  a  few  instances 
in  which  the  stimulative  effect  of  fertilization  is  not  necessary, 
as  in  seedless  Oranges,  seedless  Persimmons,  Bananas,  and  a 
few  other  fruits  known  as  parthe7iocarpic  fruits,  which  develop, 
although  no  fertilization  occurs.  There  are  a  few  plants,  the 
Dandelion  being  a  common  one,  in  which  ovules  develop  into 
seeds  parthenogeneticalhj ,  that  is,  without  fertilization,  but  such 
plants  as  well  as  those  that  develop  seedless  fruits  are  exceptional. 
In  most  cases  our  harvest  of  seed  and  fruit  depends  upon  the 
stimulative  effect  of  fertilization. 

1  Research  Bulletin  4,  Wisconsin  Agr.  Exp.  Sta.,  1909. 


THE    RESULTS  OF   POLLINATION 


51 


The  effect  of  fertilization  in  reference  to  the  influence  which 
the  sperms  have  upon  the  character  of  the  endosperm  of  the  seed 
and  upon  the  character  of  the  plant  which 
the  embryo  of  the  seed  will  produce  is  a  sub- 
ject receiving  much  attention  in  plant-breed- 
ing. The  endosperm  nucleus  consists  of  a 
sperm  and  a  primary  endosperm  nucleus, 
each  of  which  is  capable  of  determining  the 
character  of  the  endosperm.  Likewise,  in 
the  fertihzed  egg,  the  contents  of  both  sperm 
and  egg  are  capable  of  determining  the 
characteristics  of  the  plant  developing  from 
the  fertilized  egg.  But  the  influence  of 
the  sperm  is  toward  the  production  of  both 
endosperm  and  plants  having  the  features 
which  are  characteristic  of  the  pollen  parent, 
while  the  egg  and  primary  endosperm 
nucleus  tend  to  reproduce  in  the  offspring 
those  features  characteristic  of  the  mother 
plant.  Thus  it  follows  that  if  the  pollen 
parent  is  very  different  from  the  mother 
plant,  as  is  the  case  when  the  parents  be- 
long to  different  varieties  or  species,  there 
will  be  opposing  tendencies  in  the  fertilized 
egg  and  endosperm  nucleus.  Such  a  fertil- 
ized egg  develops  into  a  plant  known  as  a 
hybrid.  The  hybrid  character  of  the  endo- 
sperm in  most  seeds  is  either  lost  through  Sweet  Corn  showing 
the    absorption    of    the    endosperm  bv  the  the  effect  of  the  pollen 

embryo  or  obscured  by  coverings.     It""  is  in  ^  Y^"°,^  ^^^*,  ^^"^ 

^,       „  ^  PI  ■      r^  1  The     plump     kernels 

the  Grass  type  of  seeds,  as  m  Corn  where  ^ave   endosperm   like 

the  endosperm  remains  outside  of  the  em-  the  Yellow  Dent  Corn, 

bryo  and  can  be  seen  through  the  pericarp,  due  to  the  mfluence  of 

that   the   influence    of    the    sperm   on    the  the  sperm  which  fused 

endosperm    and    known    as   xenia   is    often  with  the  primary  endo- 


Fig.  57.  —  An  ear  of 


sperm  nucleus.      After 
H.  J.  Webber. 


noticeable.  Notice  the  ear  of  Corn  shown 
in  Figure  57.  This  was  an  ear  of  Sweet 
Corn  which  was  partly  pollinated  with  pollen  from  hard  Field 
Corn.  Notice  the  kernels  which  have  the  hard  plump  endo- 
sperm and  resemble  the   kernels  of  Field  Corn.      In   the   dc- 


52  PISTILS   AND   STAMENS 

velopment  of  these  kernels,  the  sperm  portion  of  the  endo- 
sperm nucleus  dominated,  and  thus  the  endosperm  is  like  the 
endosperm  of  the  pollen  parent.  The  sperm  may  even  deter- 
mine the  color  and  fat  content  of  the  endosperm.  On  the  other 
hand,  if  Field  Corn  is  pollinated  with  pollen  from  Sweet  Corn, 
then  usually  the  primary  endosperm  nucleus  dominates  and  one 
sees  no  effect  of  the  sperm.  Thus  it  is  seen  that  the  character 
of  the  endosperm  of  a  seed  may  be  determined  by  either  of  the 
members  which  fused  in  forming  the  endosperm  nucleus. 

The  kernels  in  Figure  57  which  have  the  endosperm  features 
of  Field  Corn    also    have  embryos   with  opposing  tendencies. 


Fig.  58.  —  Pears  showing  a  difference  between  the  results  of  self-  and  cross- 
pollination,  a,  fruit  resulting  from  self-pollination;  6,  fruit  resulting  from 
cross-pollination.     After  Waite. 

These  embryos  received  from  the  egg  tendencies  to  develop  into 
plants  having  all  of  the  features  of  Sweet  Corn.  They  also  re- 
ceived from  the  sperm  tendencies  to  develop  plants  having  all 
of  the  features  of  Field  Corn.  In  the  hybrid  offspring  it  is  likely 
that  some  of  the  characters  of  both  parents  will  be  present. 

The  Kind  of  Pollination  Giving  the  Best  Results.  —  Plants  in 
general  seem  to  favor  cross-pollination  and  often  have  their 
flowers  so  constructed  as  to  prevent  self-pollination.  In  some 
plants,  however,  as  in  the  small  grains,  Beans,  Peas,  and  some 
other  plants,  self-pollination  is  the  usual  method  and  gives  good 
results.  Red  clover,  many  fruit  trees,  and  many  other  plants 
require  cross-pollination  and  will  develop  very  little  seed  or  fruit 


THE   KIND  OF   POLLINATION   GIVING    BEST  RESULTS      53 

when  self-pollinated.  Many  of  our  Pears,  such  as  the  Anjou, 
Bartlett,  Pound,  Lawrence,  Jones,  Howell,  Sheldon,  Wilder,  and 
some  others  will  not  produce  much  fruit  unless  pollinated  with 
pollen  from  other  varieties,  while  the  Kiefer,  Buffum,  Seckel,  and 
some  others  known  as  self-fertile  varieties  set  fruit  well  when 
self-pollinated.  Moreover,  some  trees  which  are  self-fertile 
develop  larger  and  better  fruit  when  cross-pollinated.  (Fig.  58.) 
Many  of  our  Apple  trees  and  Cherry  trees  are  known  to  require 
cross-pollinat  ion . 

Furthermore,  some  varieties  of  fruit  trees  ^  which  require  cross- 


FiG.  59.  —  Results  of  cross-pollination  with  different  varieties  in  the 
Sweet  Cherry.  A,  fruit  obtained  by  pollinating  a  cluster  of  flowers  of  the 
Bing  with  pollen  from  the  Black  Republican.  B,  fruit  obtained  by  polli- 
nating a  cluster  of  flowers  of  the  Bing  with  pollen  from  the  Knight.  After 
V.  R.  Gardner. 

pollination  will  not  do  equally  well  when  crossed  with  all  varieties. 
In  Apples,  Pears,  and  Cherries  better  results  have  been  obtained 

^  The  poUination  of  pear  flowers.  Bulletin  5,  Div.  of  Veg.  Path.,  U.  S. 
Dept.  of  Agr.,  1894. 

Pollination  of  the  apple.     Bulletin  104,  Oregon  Agr.  College  Exp.  Sta.,  1909. 

Pollination  of  the  Sweet  Cherry.  Bulletin  116,  Oregon  Agr.  College  Exp. 
Sta.,  1913. 

Read  Pollination  in  Orchards.  Bulletin  1S7,  Cornell  University  Exp.  Sta., 
1909.  Also  Pollination  of  Bartlett  and  Kiefer  Pear.  Ann.  Report,  Virginia 
Agr.  Exp.  Sta.,  1911. 


54 


PISTILS  AND  STAMENS 


by  crossing  with  some  varieties  than  with  others.  {Fig.  59.)  In 
case  of  Sweet  Cherries,  when  flowers  of  the  Bing,  a  variety  requir- 
ing cross-polhnation,  were  polHnated  with  pollen  from  the  variety 
called  the  Knight,  only  a  few  fruits  developed;  while  flowers 
pollinated  with  pollen  from  the  Black  Republican  produced 
fruit  abundantly.  Obviously  much  of  the  success  in  orcharding 
has  to  do  with  securing  for  each  variety  of  fruits  the  best  kind  of 
pollination. 


CHAPTER  V 

SEEDS   AND   FRUITS 

Nature  and  Structure  of  Seeds 

The  seed  is  the  principal  structure  by  which  plants  increase 
in  number.  The  chief  function  of  a  seed  is  to  produce  a  plant 
like  the  one  that  bore  it.  For  plants  to  increase  in  number  and 
at  the  same  time  thrive  well,  they  must  spread  to  new  areas. 
Seeds  are  thus  so  constructed  that  they  can  separate  from  the 
parent  plant  and  be  carried  to  regions  where  there  is  opportunity 
for  new  plants  to  develop.  Seeds,  being  able  in  a  dormant  state 
to  Uve  long  and  endure  adverse  conditions,  are  the  means  by 
which  those  plants  Uving  only  one  season  are  able  to  perpetuate 
themselves.  As  to  origin  the  seed  is  sometimes  defined  as  a 
matured  ovule,  that  is,  it  is  an  ovule  in  which  three  things  have 
taken  place:  (1)  the  fertilized  egg  has  developed  into  an  embryo, 
the  miniature  plant  of  the  seed;  (2)  the  fertihzed  primary  endo- 
sperm nucleus  with  some  adjacent  protoplasm  has  produced  a 
mass  of  stored  food  or  endosperm;  and  (3)  the  outer  portions  of 
the  ovule  have  been  modified  into  a  testa  or  seed  coat.  Despite 
a  wide  variation  in  size,  shape,  color,  and  other  external  features, 
seeds  possess  in  common  an  embryo,  stored  food,  and  seed  coat. 
In  many  cases  these  three  parts  are  not  separate,  for  the  endo- 
sperm may  be  absorbed  by  the  embryo  during  the  development 
of  the  seed.  This  is  true  in  the  Bean,  Pumpkin,  and  a  number 
of  other  families,  where  the  seeds  consequently  have  only  two 
distinct  parts,  embryo  and  testa. 

Each  part  of  the  seed  has  a  distinct  function  to  perform.  The 
embryo  develops  into  a  new  plant,  the  reserve  food  nourishes  the 
young  plant  until  roots  and  leaves  are  established,  and  the  seed 
coat  protects  the  embryo  and  endosperm  during  the  resting  stage 
of  the  seed.  It  is  due  to  the  embryo  that  seeds  are  valuable 
in  the  production  of  new  plants,  while  the  stored  food  makes 
many  seeds  valualole  food  for  animals. 

The  embryo,  which  is  the  chief  structure  of  the  seed,  is  the 
55 


56  SEEDS  AND   FRUITS 

young  plant,  which  after  reaching  a  certain  stage  of  development, 
varying  in  different  plants,  passes  into  a  dormant  stage  from  which 
it  may  awake  if  conditions  are  favorable  and  continue  its  devel- 
opment until  it  becomes  a  mature  plant.  In  the  development  of 
the  embryo  from  the  fertilization  of  the  egg  to  the  dormant  stage, 
certain  structures  which  function  in  the  further  development  of 
the  young  plant  are  usually  more  or  less  developed.  In  a  well 
formed  embryo  like  that  of  the  Bean,  there  are  four  parts,  hyocotyl, 
plumule,  cotyledons,  and  radicle.  In  Figure  60  of  the  Bean,  h  is 
hypocotyl,  p,  plumule,  and  c,  cotyledons.  The  radicle  (r)  is  at 
the  lower  end  of  the  hypocotyl  and  is  so  closely  joined  with  the 
hypocotyl  that  it  does  not  appear  as 
a  separate  structure.  The  cotyledons 
of  the  Bean  have  absorbed  the  endo- 
sperm and  consequently  are  so  much 
enlarged  that  they  form  the  bulk  of 
the  embryo.  The  special  functions 
performed  by  the  different  parts  of  the 
embryo  are  quite  noticeable  in  the 
germination  of  the  seed.  The  cotyle- 
dons supply  food;  the  plumule  develops 
Fig.  60.  Bean  with  testa  gtem  and  leaves;  the  radicle  develops 
removed    and    cotyledons  ,  ij.ii,  j.    i    • 

,         ,  f  ,    ,  a  root     and  the   hypocotyl    in  many 

spread  apart,     c,  cotyledons;  n      i  i     ,  ,     ,  / 

h,  hypocotyl;  p,  plumule;  r,     ^ases  pulls  the  cotyledons  and  plumule 
radicle.  out  of  the  seed  coat  and  raises  them 

above  ground. 
The  stored  food  and  seed  coat  are  temporary  structures.  They 
nourish  and  protect  the  young  plant  in  its  early  stage  of  develop- 
ment and  then  disappear.  The  stored  food,  consisting  chiefly  of 
starch,  proteins,  and  oils,  the  proportion  varying  in  different 
seeds,  develops  in  close  contact  with  the  embryo  and  when  not 
absorbed  as  rapidly  as  it  develops,  it  forms  the  storage  tissue  or 
endosperm  in  which  the  embryo  becomes  imbedded.  The  testa, 
the  protective  structure  of  the  seed  and  usually  formed  from 
the  integuments  of  the  ovule,  generally  consists  of  a  single 
covering  so  much  thickened  and  hardened  that  it  protects  the 
embryo  against  injuries.  Often  there  is  a  thin  inner  covering 
and  in  exceptional  seeds,  like  those  of  the  Water  Lily,  an  extra 
outer  covering  called  the  aril  develops  later  than  the  integuments 
and  forms  a  loose  covering  about  the  seed.     {Fig.  62.) 


NATURE   AND  STRUCTURE  OF  SEEDS 


57 


On  the  surface  of  seeds  occur  certain  structures  which  suggest 
the  structural  relation  of  the  seed  to  the  ovule.  The  micropyle, 
the  small  opening  through  which  the  pollen  tube  entered  the 
ovule,  persists  as  a  tiny  pit  on  the  seed  coat.  Usually  near  the 
micropyle  there  is  a  much  larger  scar,  called  the  hilu7n,  left  where 
the  seed  broke  away  from  the  funiculus,  the  stalk-like  structure 
which  attached  the  ovule  to  the  ovary  and  through  which  the 
seed  received  food  and  water  during  its  development.  (Fig.  61.) 
In  case  an  ovule  turns  over  on  its  elongated  stalk  and  grows  fast 
to  it,  the  stalk  persists  on  the 
seed  coat  as  a  distinct  ridge, 
called  the  raphe.  {Fig.  62.) 
In  some  seeds,  like  those  of 


Fig.  61.  —  Beans  showing  the 
hylum  at   h   and   the    micropyle 

at  m. 


A  B  ^- 

Fig.  62.  —  A,  seed  of  Pansy  showing 
raphe  (r) .  B,  seed  of  Castor  Bean  show- 
ing caruncle  (c).  C,  seed  of  White 
Water  Lily  showing  the  aril  or  loose 
jacket  around  the  seed. 


the  Castor  Bean,  an  enlargement  known  as  the  caruncle  develops 
near  the  micropyle. 

Structures  such  as  hairs,  plumes,  hooks,  and  other  appendages 
which  do  not  occur  on  ovules,  are  direct  outgrowths  of  the  seed 
coat  and  function  chiefly  in  dissemination.  Similar  appendages 
occur  often  on  one-seeded  ovaries  in  which  case  one  can  tell  only 
by  dissection  whether  the  structure  is  a  seed  or  one-seeded  fruit. 

Many  of  the  small  one-seeded  fruits  are  commonly  called  seeds. 
In  addition  to  a  seed,  they  contain  the  ovary  wall  which  persists 
as  an  outer  covering  over  the  seed.  The  so-called  seeds  of  Let- 
tuce, Buckwheat,  Ragweed,  and  the  grains  such  as  Corn,  Wheat, 
Barley,  Rye,  and  Oats  are  familiar  examples  of  one-seeded  fruits 
which  are  commonly  called  seeds.  While  they  are  not  identical 
with  true  seeds  in  structure,  they  are  in  function  and  therefore 
may  be  appropriately  discussed  with  seeds.  In  these  one-seeded 
fruits,  the  seed  is  protected  by  the  hardened  ovary  wall,  and 
consequently,  the  seed  coat  is  poorly  developed,  forming  only  a 


58  SEEDS  AND   FRUITS 

thin  covering,  which  is  usually  tightly  pressed  against  the  inner 
side  of  the  ovary  wall. 

In  general  structure  seeds  are  similar,  all  having  an  embryo, 
stored  food,  and  seed  coat,  but  in  size,  shape,  and  in  features 
which  pertain  to  the  structure  of  the  embryo,  composition  of  the 
stored  food,  and  character  of  the  seed  coat,  seeds  vary  widely 
and  can  be  used  in  many  ways  by  man.  The  number  of  coty- 
ledons developed  by  the  embryo  is  used  as  a  basis  upon  which  to 
classify  the  Flowering  Plants  into  two  classes.  Monocotyledons 
and  Dicotyledons.  From  the  stored  food,  whether  stored  as 
endosperm  or  in  the  embryo,  various  valuable  products,  such  as 
starch,  protein,  fats  and  oils,  are  obtained;  and  from  the  hair- 
like outgrowth  of  the  seed  coat,  as  in  case  of  Cotton,  various  fiber 
products  are  made.  Although  seeds  may  be  divided  into  many 
types  on  the  basis  of  their  structure  and  external  features,  only 
those  types  which  include  the  most  common  seeds  will  be  studied 
in  this  presentation. 

Bean  Type  of  Seeds.  —  Of  this  type  of  seeds,  those  of  the 
Bean,  Pea,  Peanut,  Clover,  Vetch,  Alfalfa,  Cotton,  Pumpkin, 
Squash,  Melon,  Apple,  Peach,  Oak,  Hickory,  and  Walnut  are  ex- 
amples. The  type  is  so  named  because  it  is  characteristic  of  the 
Bean  family  (Legiiminosae) ,  a  family  notable  for  its  many  valu- 
able cultivated  forms  among  which  are  Clover,  Alfalfa,  Beans, 
Peas,  Vetch,  and  Peanuts.  The  type  is  also  characteristic  of  the 
Rose  family  (Rosaceae),  the  family  to  which  most  fruits,  such  as 
the  Apple,  Peach,  Pear,  Cherry,  etc.  belong.  In  this  family, 
however,  it  is  the  fruit  (rarely  the  seed)  that  is  important. 
The  seeds  of  the  Bean  type  are  common  to  a  number  of 
plant  families  and  to  species  and  varieties  of  plants  so  numer- 
ous that  a  list  naming  them  all  would  require  a  page  or  more. 
Although  many  are  valuable  commercial  seeds,  some  are  borne 
by  weeds  and  hence  of  interest  because  of  their  undesirable 
features. 

These  seeds  differ  from  other  types  in  having  little  or  no  endo- 
sperm. As  the  seed  develops,  all  or  almost  all  of  the  food  tis- 
sue formed  by  the  endosperm  nucleus  and  adjacent  cytoplasm 
is  absorbed  by  the  embryo  where  it  is  stored  in  the  cotyledons, 
which,  consequently,  are  so  much  enlarged  that  they  are  much 
the  largest  part  of  the  embryo.  {Fig.  63.)  For  this  reason 
these  seeds  are  called  exalhuminous  seeds,  that  is,  seeds  without 


SEEDS  OF  THE   BUCKWHEAT  AND   FLAX  TYPE 


59 


endosperm.     Another  feature  to  be  noted  is  that   the  embryo 

has  two  cotyledons. 

In  external  characters  they  vary  so  much  that  their  type  in 

most  cases  can  be  determined  only  by  an  examination  of  their 

structure.     In  size,  those  most  commonly 

grown  in  our  region  vary  from  the  smallest 

of  the  Clover  Seeds  up  to  the  largest  of 

the  Beans.  They  are  kidney-shaped,  glob- 
ular, oval,  or  flattened.  Among  them  vari- 
ous   colors    such    as    red,    purple,    brown, 

yellow,    green,   mottled,   and  black   occur. 

In  identifying  the  different  seeds   of  this 

type,  especially  those  of  the  Bean  family, 

size,  shape,  and  color  are  important  aids. 
In   importance,  the   seeds   of  this   type 

rank  next  to  those  of  the  Grass  family.    In 

Beans,  Peas,  and  Peanuts,  which  are  used 

directly  as  food,  the  value  depends  upon 

the  protein,  fats,  and  starches  stored  in  the  seed  sectioned  longitudi 
embryo.     In  the    nally.      B,    Apple    seed 
^>5''^"^        0*^N  Cotton     seed    sectioned  longitudinally. 

■,       ,  ,  1  e,  embryo.   B  much  more 

bothembryo       ,       j^,        . 
"^        enlarged  than  A . 

and    seed     coat 

are  valuable  structures.  The  embryo 
is  rich  in  oil  from  which  many  useful 
products  are  made,  while  the  hairs  of 
the  seed  coat  are  the  Cotton  fibers 
of  commerce.  (Fig.  64-)  The  seeds 
of  Clover  and  Alfalfa  are  important 
liecause  the  plants  which  bear  them 
Fig.  64.  —  Section  through  increase  the  soil  fertility  and  are  valu- 
a   Cotton   seed   showing   the  able  for  hay  and  forage. 


A  B 

Fig.  63.— a,  Squash 


embryo  with  its  much  folded 
cotyledons,  and  the  seed  coat 
with  the  seed  hairs.  Enlarged 
about  twice. 


Seeds  of  the  Buckwheat  and  Tomato 
Type.  —  Some  common  seeds  of  this 
type  are  those  of  the  Buckwheat,  Beet, 
Tomato,  Potato,  Tobacco,  Red  Pep- 
per, Coffee,  Flax,  and  Castor-oil  plant.  The  type  is  common  to 
a  number  of  families,  which  contain  some  useful  plants  and  many 
weeds,  such  as  Nightshades,  Spurges,  Morning  Glories,  Bind- 
weeds, Dodders,  Milkweeds,  Docks,  Smartweeds,  and  Corn  Cockle. 


60 


SEEDS  AND    FRUITS 


em 


B 


65.  —  A,   section   through 
c,    embryo;    e,    endosperm; 


a   Potato 
t,   testa. 


In  structure  these  seeds  differ  from  those  of  the  Bean  type  in 
that  they  have  three  distinct  parts,  an  embryo,  endosperm,  and 
seed  coat,  but  in  number  of  cotyledons,  which  is  two,  the  two 
types  are  identical.  Since  endosperm  is  present,  the  seeds  of 
this  type  are  known  as  albuminous  seeds.  {Fig.  65.)  Although 
some  endosperm  is  always  present,  sometimes,  however,  much 
of  it  is  absorbed  by  the  embryo  during  the  development  of  the 
seed,  and  in  this  case  the  cotyledons,  which  are  comparatively 

free  from  stored  food  in 
many  of  these  seeds,  as- 
sume some  importance  as 
storage  organs,  though  not 
so  much  as  in  the  Bean 
type.  In  the  Buckwheat 
family,  represented  by 
Buckwheat,  Rhubarb, 
Docks,  and  Smart  weeds, 
and  also  in  some  plants 
of  the  Goosefoot  family, 
the  hardened  ovary  wall, 
which,  when  mature  re- 
sembles a  seed  coat,  per- 
sists as  an  outer  covering  over  the  seed,  thus  forming  with  the 
seed  a  fruit-like  structure  known  as  an  achene,  a  term  which  is 
applied  to  many  hard,  usually  one-seeded  fruits,  that  do  not  dehisce 
or,  in  other  words,  that  do  not  open  to  allow  the  seed  to  escape. 
In  external  characters,  seeds  of  this  type  present  various  differ- 
ences by  means  of  which  one  can  usually  identify  the  family  and 
often  the  species  to  which  the  seed  belongs.  Those  most  com- 
mon in  our  region  range  in  size  from  the  smallest  of  the  Dodder 
seeds,  which  are  almost  dust  fine,  to  the  size  of  the  Castor 
Bean.  The  shape,  which  in  many  cases  is  the  chief  character  by 
which  the  family  and  often  the  species  to  which  the  seed  belongs 
is  identified,  may  be  globular,  oval,  flat,  or  angled.  Such  colors 
as  red,  yellow,  brown,  and  black  are  common  and  serve  along 
with  shape  and  size  as  a  means  of  identifying  different  seeds. 
Sometimes  the  seed  coat  is  much  roughened,  as  in  the  Cockle, 
and  in  some  cases,  as  in  the  Milkweeds,  the  seed  coat  develops 
hair-like  appendages. 

In  case  of  Flax,  Buckwheat,  Coffee,  and  the  Castor-oil  plant. 


B,  section  through  an  achene  of  Buckwheat. 
em,  embryo;  e,  endosperm;  o,  ovary  wall 
and  testa.     Enlarged. 


GRASS  TYPE  OF  SEEDS  61 

the  seeds  themselves  are  valuable  on  account  of  the  oil,  protein, 
starch,  or  alkaloid-like  substances  which  they  contain.  From  the 
endosperm  and  embryo  of  the  Flax  seed,  linseed  oil,  the  chief  sol- 
vent for  paints,  is  obtained.  After  the  oil  is  pressed  out  of  the 
flax  seed,  there  remains  the  cake,  which  has  considerable  value  as 
a  feed  for  stock.  The  Castor  Bean  yields  castor-oil  which  is 
much  used  as  a  medicine  and  sometimes  as  a  lubricant  and  illu- 
minant.  Buckwheat,  which  contains  much  starch  and  some  fat 
and  protein,  is  much  used  for  food  when  ground  into  flour.  Often, 
as  in  case  of  the  Tomato,  Potato,  Beet,  and  Tobacco,  the  value 
of  the  plants  depends  upon  the  fruit,  tubers,  roots,  or  leaves,  and 
not  upon  the  seed,  which  in  these  cases  has  no  value  except  for 
growing  new  plants.  Of  the  weed  seeds  of  this  type,  some  com- 
monly occur  as  impurities  among  the  seeds  of  Clover,  Alfalfa, 
Flax,  and  the  small  grains  and,  when  present  in  considerable  quan- 
tities, they  either  lower  the  price  or  prevent  the  sale  of  these  agri- 
cultural seeds,  thus  bringing  loss  to  the  farmer.  In  case  of  Cow 
Cockle  and  Corn  Cockle,  the  seeds,  which  are  frequently  found 
among  the  small  grains,  are  poisonous  and  when  ground  with 
Wheat  make  the  flour  unwholesome  and  when  fed  with  grain  to 
stock  often  cause  injury.  Other  weed  seeds  of  this  type,  as  those 
of  Dodder,  Morning  Glories,  Black  Bindweed,  Sheep  Sorrel,  and 
others  are  objectionable  because  the  plants  themselves  hinder 
the  cultivation  and  growth  of  useful  plants.  Sometimes,  as  in 
case  of  the  Black  Nightshade  and  Jimson  Weed,  the  plants  are 
poisonous. 

Grass  Type  of  Seeds.  —  As  the  name  suggests,  these  are  the 
seeds  of  the  Grass  family,  the  family  to  which  Corn,  Wheat,  Oats, 
Rye,  Barley,  and  Rice  belong  and  hence  the  family  most  depended 
upon  for  food.  Many  of  the  Grass  seeds,  as  in  case  of  Timothy, 
Red  Top,  Blue  Grass,  etc.,  though  not  used  for  food,  are  valuable 
because  the  plants  themselves  are  useful  for  pasture  and  hay. 
Some  of  the  Grasses,  however,  are  regarded  as  weeds  and  their 
seeds  are  often  troublesome  impurities  among  agricultural  seeds. 

As  previously  noted,  in  structure  the  seeds  of -the  Grass  type 
are  not  true  seeds.  Besides  a  seed,  they  contain  the  ovary  wall, 
called  the  pericarp,  which  remains  about  the  seed  as  a  closely 
fitting  jacket.  They  are  one-seeded  ovaries  and  hence  struc- 
turally they  are  fruits  rather  than  seeds.  Although  popularl}^ 
known  as  a  seed,  this  fruit-like  structure  of  the  Grasses  is  scien- 


62  SEEDS   AND   FRUITS 

tifically  called  a  cariopsis,  a  term  which  refers  to  its  nut-like  char- 
acter. 

The  seed  itself  contains  three  distinct  parts,  embryo,  endo- 
sperm, and  seed  coat.  The  seed  coat,  however,  since  it  is  covered 
by  the  ovary  wall  which  performs  the  protective  function  of  an 
ordinary  seed  coat,  is  poorly  developed  and  so  closely  joined 
with  the  ovary  wall  that  it  appears  to  be  a  part  of  its  structure. 
In  containing  three  distinct  parts,  embryo,  endosperm,  and  seed 
coat,  it  is  seen  that  the  seeds  of  the  Grass  type  are  identical  with 
those  of  the  Flax  and  Buckwheat  type:  but  in  possessing  only 
one  cotyledon  instead  of  two,  they  are  clearly  distinguished 
from  both  of  the  other  types. 

In  external  features,  the  seeds  of  the  Grasses  present  many 
variations,  though  probably  not  so  many  as  occur  among  some 
other  types  of  seeds.  Most  of  them  are  small,  but  various  sizes, 
ranging  from  that  of  a  Timothy  seed  and  even  smaller  up  to 
that  of  a  Corn  kernel  occur.  In  most  cases  they  are  elongated, 
and  have  a  groove  on  one  side.  In  most  varieties  of  Oats  and 
Barley,  and  in  many  of  the  Grasses  having  very  small  seeds,  the 
cariopsis  remains  enveloped  by  the  palea  and  flowering  glume,  in 
which  case  the  entire  structure  may  have  the  appearance  of  a 
seed,  especially  when  the  barbed  awns  and  other  structures  devel- 
oped by  the  flowering  glume  function  in  dissemination.  The 
seeds  of  most  Grasses  are  white,  gray,  yellow  or  brown,  but  in 
Corn  such  colors  as  red,  blue,  purple,  and  black  often  occur. 

The  seeds  of  those  Grasses  known  as  the  grains  are  our  chief 
source  of  food.  Although  all  of  the  grains  contain  practically 
the  same  food  elements,  they  differ  in  the  proportion  of  the  differ- 
ent elements  and  consequently  are  fitted  for  different  uses.  Even 
within  a  seed,  various  structures  differ  so  much  in  composition 
that  they  are  adapted  to  special  uses  as  is  well  shown  in  the 
milling  of  Wheat.  Likewise  in  case  of  Corn,  the  oil  and  protein 
content  are  so  closely  related  to  structure  that  one  can  judge  the 
relative  proportion  of  these  substances  by  observing  the  relative 
sizes  of  certain  structures  of  the  kernel. 

Com  Kernel.  —  A  study  of  a  section  through  a  kernel  of  corn, 
as  shown  in  Figure  66,  will  give  a  notion  of  the  general  structure 
of  the  Grass  type  of  seeds.  Notice  that  within  the  covering  (a) 
there  are  two  distinct  regions,  that  to  the  right  and  below  being 
the  embryo,  and  that  to  the  left  and  above  being  the  endosperm. 


CORN   KERNEL 


63 


The  location  of  the  embryo  at  one  side  of  the  endosperm,  instead 
of  being  centrally  located  and  surrounded  by  the  endosperm,  is  a 
peculiar  feature  of  the  Grass  type  of  seeds. 

The  embryo  consists  of  two  main  parts:  the  large  scutellum  or 
cotyledon  (cot)  which  lies  in  con- 
tact with  the  endosperm,  and  the 
embryonic  axis  which  upon  germi- 
nation produces  the  stem  at  its 
upper  and  roots  at  its  lower  end. 
The  axis  is  attached  along  its 
central  region  to  the  cotyledon, 
which  supplies  it  food  during 
growth.  At  the  upper  end  of  the 
axis  is  the  plumule,  a  small  bud- 
hke  structure  consisting  of  a  grow- 
ing point  (gr)  and  some  small 
leaves  (l).  The  plumule  is  en- 
closed in  a  sheath  (d)  called  col- 

eoptile.       Between    the    plumule  ^     '                 .       ,        ,     , 

1   , ,          , ,      1           j_      r  xi           X  Fig.  66.  —  Section  through  a  ker- 

and  the  attachment  oi  the  coty-  i   r  n            ,      ^  ^  a 

•^  nel  of  Corn,    cot,  cotyledon;  ep,  epi- 

ledon  is  a  short  stem  {st),  which  theUal  layer  of  cotyledon;  d,  coleop- 

with  the  plumule   is  often    called    tile;  gr,  growing  point  of  plumule; 

epicotijl    (the    portion    above    the   ^,  young  leaves;  s/,  epicotyl;  r,  rach- 

cotyledon).      The  portion  of  the  ^^^J^  [!'/?°L!!!'J,  Zl^f"!^^^.!! 

axis  below  the  cotyledon  consists 

chiefly  of  the  radicle  (r),  the  struc-  enlarged, 

ture  which  develops  the  first  root. 

The  radicle  bears  at  its  tip  the  root  cap  (re)  and  is  enclosed  by 

the  coleorhiza  (cr). 

The  hypocotyl,  which  is  all  or  only  a  part  of  the  axis  between  the 
plumule  and  radicle  (a  point  in  dispute  among  botanists),  is  the 
portion  of  the  axis  developing  least  when  the  embryo  resumes 
growth.  In  the  Grasses  there  is  very  Httle  elongation  of  the  hypo- 
cotyl and,  consequently,  the  establishment  of  the  young  plant  in 
the  soil  and  light  depends  mainly  upon  the  growth  of  the  radicle 
and  plumule.  The  fact  that  the  hypocotyl  remains  small  while 
the  radicle,  since  it  forms  the  first  root,  becomes  a  prominent 
structure,  accounts  for  the  general  application  of  the  term  rad- 
icle to  all  of  the  lower  portion  of  the  axis,  and  the  rare  use  of  the 
term  hypocotyl  in  connection  with  grass  embryos. 


soft  endosperm;  h,  hard  endosperm; 
covering  called  pericarp.     Much 


64  SEEDS   AND   FRUITS 

Concerning  the  cotyledon  of  the  Grass  embryo,  there  is  some 
dispute.  Some  morphologists  regard  the  scutellum  as  the  coty- 
ledon, while  others  think  that  the  cotyledon  includes  both  the  scu- 
tellum and  coleoptile.  Although  the  cotyledon  may  include 
other  structures,  the  scutellum,  in  absorbing  and  supplying  food 
to  the  growing  parts  of  the  embryo,  performs  the  function  of  a 
cotyledon.  The  scutellum  is  a  boat-shaped  structure  with  its 
keel-like  portion  imbedded  in  the  endosperm.  Its  broad  side 
bearing  the  axis  of  the  embryo  is  visible  through  the  testa  and 
ovar>^  wall.  The  keel-like  portion  is  covered  with  specialized 
cells  formed  into  a  layer  called  the  epithelium.  The  epithelium 
secretes  soluble  substances  called  enzymes,  which  after  diffusing 
to  the  endosperm  change  the  foods  stored  there  into  soluble 
forms,  which  are  then  absorbed  by  the  cotyledon  and  carried  to 
the  plumule  and  radicle  where  they  are  used  for  growth. 

The  principal  food  substances, 
stored  in  the  endosperm  are  starch, 
fat,  and  protein.    Although  occur- 
ring together  in  most  parts  of  the 
^  B  endosperm,     each     substance     is 

Fig.  67.  — Kernels  of  Corn  with  present   in   a   greater  proportion 
high  and  with  low  percentages  of  ^^  ^^^^  regions  than  in  others, 
protein.     A,  kernel  with  high  per-        ™,  „  j  j.v,     u      j         c 

.  c  ,.  ■  u  ^  1  uu  I  he  cells  around  the  border  ot 
centage  of  protein.     B,  kernel  with 

low  percentage  of  protein,  a,  horny  the  endosperm  and  forming  the 
endosperm;  h,  white  starchy  endo-  oleuron  layer  are  especially  rich  in 
sperm;  e,  embryo.  Aher  Bulletin 87 ,  protein,  which  is  present  in  the 
University  of  Ilhnois  Agricultural  ^^^^  ^f  granules  and  SO  abundant 
Experiment  Station.  xu    i.    xi.  n  j 

that    the   cells   appear    as    dense 

granular  masses.  The  remaining  endosperm,  which  is  especially 
rich  in  starch,  consists  of  two  regions.  The  outer  region  (more 
deeply  shaded)  is  the  horny  endosperm  (h)  and  contains  much 
protein  in  addition  to  starch.  The  inner  region  (n)  (with  lighter 
shading)  is  the  starchy  endosperm,  which  is  not  only  much  softer 
and  more  granular  than  the  horny  endosperm  but  also  contains 
less  protein.  The  richness  ^  of  the  kernel  in  protein  depends  so 
much  upon  the  amount  of  horny  endosperm  that  by  cutting  across 
a  kernel  as  shown  in  Figure  67,  one  may  judge  the  richness  of  the 
kernel  in  protein  by  observing  the  relative  amount  of  the  two 
kinds  of  the  endosperm.  Likewise,  since  the  embryo  of  the  ker- 
1  See  Bulklins  44,  82,  and  87,  University  of  Illinois  Agricultural  Experiment 
Station. 


GRAIN   OF   WHEAT 


65 


nel  contains  most  of  the  oil,  the  oil  content  depends  largely  upon 
the  size  of  the  embryo.  (Fig.  68.)  Sometimes,  however,  much 
of  the  starch  of  the  endosperm  is  replaced  by  sugar,  as  in  case 
of  Sweet  Corn,  which  is  much  used  as  a  vegetable  on  account  of 
its  soft  sweet  endosperm. 

Grain  of  Wheat.  —  In  structure,  a  grain  of  Wheat  is  similar  to 
a  kernel  of  Corn.  In  the  section  through  a  Wheat  grain,  shown 
in  Figure  69,  though  the  parts  are  not  labelled,  they  can  be  deter- 
mined by  referring  to  the  section  of  the  Corn  kernel  shown  in 


Fig.  68.  —  Kernels  of  Corn 
with  high  and  with  low  per- 
centage of  oil.  A,  kernel  with 
large  embryo  and  hence  rich 
in  oil.  B,  kernel  with  small 
embryo  and  low  percentage  of 
oil.  C  and  D,  face  views  of 
two  kernels  differing  in  size  of 
embryos  and  therefore  in  oil 
content,  e,  embryo.  After 
Bulletin  87,  University  of  Illi- 
nois Agricultural  Experiment 
Station. 


Fig.  69.  —  Lengthwise  section 
through  a  Wheat  kernel.  The 
embryo  is  to  be  compared  with 
the  embryo  of  the  Corn  kernel 
{Fig.  66)  and  parts  labelled. 


Figure  66.  In  milling  i  a  grain  of  Wheat,  a  number  of  special 
products  are  obtained.  The  woody  pericarp  and  seed  coat 
with  the  aleuron  layer  and  some  of  the  outermost  starch  cells 
constitute  the  bran.  When  bran  is  finely  ground,  it  is  known  as 
shorts.  Middlings  differ  from  shorts  only  in  containing  a  larger 
percentage  of  starchy  endosperm.  In  making  the  best  grades  of 
flour,  only  the  starchy  endosperm  is  used  and  the  quality  of  the 

1  On  Bread.    Bulletin   4,  Ohio   Agricultural   College.     Bread  and  Bread 
Making.     Farmers'  Bulletin  389,  U.  S.  Dept.  of  Agriculture. 


66 


SEED8  AND   FRUITS 


flour  depends  much  upon  the  amount  and  quaht}^  of  protein 
(gluten)  which  the  endosperm  contains.  When  there  is  a  good 
quality  of  gluten  present,  the  flour  is  characterized  as  being  strong 
and  is  the  kind  which  bakers  prefer.  Graham  flour  is  the  entire 
grain  finely  ground.  In  making  entire  wheat  flour,  after  the 
grain  is  finely  ground,  some  of  the  bran  is  removed.  The  em- 
bryo, which  constitutes  about  eight  per  cent  of  the  grain,  contains 
much  fat  and  oil,  and,  if  the  embryo  is  ground  up  with  the  flour, 
the  oil  is  apt  to  become  rancid  and  impair  the  flavor  of  the  flour. 
For  this  reason,  the  embrj^o  is  removed  in  making  high  grade 

flour  and  sold  with  the  middlings 


or  used  in  making  breakfast  foods. 
(Fig.  70.) 

Oat  Kernel.  —  In  general  form 
and  structure  the  Oat  kernel  is 
similar  to  the  grain  of  Wheat,  ex- 
cepting that  it  is  more  elongated 
and  the  ovary  wall  is  hairy. 
The  kernel  usually  remains  en- 
closed in  the  lemma  and  palea, 
and  the  quality  of  Oats  depends 
upon  the  proportion  of  hull  to 
kernel.  The  endosperm  contains 
protein,  starch,  and  fat  and  is  a 
valuable  food  for  both  man  and 
live  stock. 

Comparison  of  Seed  Types.  — ■ 
The  three  types  of  seeds  differ 
fundamentally  in  the  number  of 
cotyledons  and  in  the  location 
of  the  stored  food.  The  difference  in  the  number  of  cotyledons 
is  probably  the  more  important  one  because  all  Flowering  Plants 
have  been  divided  into  two  classes  on  the  basis  of  whether  or  not 
one  or  two  cotyledons  are  present.  Plants  with  seeds  having 
but  one  cotyledon  are  called  Monocotyledons  (one  cotyledon). 
Not  only  the  grains  and  all  other  Grasses,  but  also  Palms,  Lilies, 
Asparagus,  Onions,  and  many  other  plants  are  Monocotyledons. 
Plants  with  seeds  having  two  cotyledons,  as  in  case  of  the  Bean, 
and  Buckwheat  type,  are  called  Dicotyledons  (two  cotyledons). 
Besides   Beans   and    Buckwheat,   many   other   common   plants, 


Fig.  70.  —  Section  through 
outer  portion  of  a  Wheat  gram,  w, 
ovary  wall,  often  called  pericarp;  t, 
testa  or  seed  coat;  a,  aleuron  layer 
with  cells  filled  with  grains  of  protein; 
g,  starchy  endosperm  with  cells  large 
and  filled  mainly  with  starch  grains. 
Some  protein  grains  are  present  in  the 
starch  cells,     n,  nucleus. 


RESTING   PERIOD 


6^ 


such  as  Peas,  Clover,  Alfalfa,  Tomatoes,  Melons,  Cotton,  Fruit 
trees,  and  many  forest  trees  are  Dicotyledons.  Each  of  these 
classes  includes  a  large  number  of  important  cultivated  plants  as 
well  as  many  that  are  regarded  as  weeds. 

Since  the  classification  into  Monocotyledons  and  Dicotyledons 
applies  only  to  the  Flowering  Plants,  such  plants  as  the  Larch, 
Pine,  Spruce,  Fir,  Hemlock,  which  belong  to  the 
Gymnosperras  where  there  are  no  true  flowers, 
are  omitted  in  this  classification.  The  seeds  of 
a  number  of  the  Gymnosperms  commonly  have 
more  than  two  and  those  of  the  Pine  and  Cypress 
commonly  have  many  cotyledons.  (Fig.  71.) 
They  are  polycotyledonous  seeds  and  the  plants 
may  be  described  as  Poly  cotyledons. 

The  difference  in  the  location  of  the  stored 
food  in  seeds  serves  in  distinguishing  them  but 
does  not  affect  their  function  or  commercial 
value.  In  all  types  of  seeds,  the  endosperm 
must  be  absorbed  by  the  cotyledons  before  it  is 
available  for  the  growth  of  the  embryo.  This 
absorption  occurs  before  germination  in  the  ex- 
albuminous  seeds  but  during  germination  in  albu- 
minous seeds.  Among  Monocotyledons  albumi-  Pi"P  seed  sec- 
nous  seeds  prevail,  while  both  types  are  about  *^°^^,'^  lengthwise 
„  T^-      ,    1    "i  to    show  polycot- 

equally  common  among  Dicotyledons.  ^  ^ 


jieaony.  h,  Pine 
seed  germinating, 
the  many  cotyle- 


free  from  the  seed 
coat.  Both  are 
enlarged. 


Resting  Period,  Vitality,  and  Longevity  of  Seeds 

The  function  of  a  seed  as  the  plant's  organ  of  dons^  becoming 
dissemination  depends  upon  a  number  of  physi- 
ological features,  of  which  the  chief  one  is  the 
ability  of  the  seed  to  remain  alive  with  the  ordi- 
nary life  processes  so  slowed  that  they  seem  not  to  be  taking 
place  at  all. 

Resting  Period.  —  The  resting  conchtion  is  a  valuable  physio- 
logical feature  to  the  seed,  because  in  this  condition  the  embryo 
can  endure  cold,  heat,  dryness,  intense  light,  and  various  other 
conditions  to  which  the  seed  is  exposed  during  dissemination. 
How  the  resting  condition  is  brought  about  and  how  it  is  main- 
tained, often  many  years,  are  not  thoroughly  understood.  The 
resting  condition  is  associated  with  dryness,  a  condition  obtained 


68  SEEDS  AND   FRUITS 

in  the  seed  by  allowing  tlie  greater  part  of  the  water  to  escape 
during  the  process  of  maturing.  Since  the  hfe  processes  depend 
upon  water  for  dissolving  and  transporting  the  necessary  sub- 
stances, they  are  naturally  slowed  down  when  water  is  with- 
drawn and  apparently  without  injury  even  when  so  checked 
that  no  action  can  be  detected  by  ordinary  laboratory  methods. 
Some  investigators  have  maintained  that  these  life  processes 
actually  stop,  but  the  evidence  sustains  the  view  that  these 
processes  never  stop  so  long  as  the  seed  remains  capable  of  ger- 
minating. There  are  various  factors  involved  in  maintaining  the 
rest  period,  but  chiefly  they  have  to  do  with  keeping  water  and 
oxygen  from  the  embryo. 

The  ability  of  seeds  to  endure  extreme  conditions  while  in  the 
resting  stage  is  well  shown  in  the  case  of  temperature.  In  hquid 
air,  seeds  of  Alfalfa,  Mustard,  and  Wheat  have  been  kept  at  a 
temperature  of  —250°  C.  for  three  days  and  afterwards  success- 
fully germinated,  though  their  embryos  when  active  are  quickly 
killed  by  a  temperature  a  little  below  freezing.  The  ability  of 
dry  seeds  to  endure  heat  is  also  surprising.  Some  in  the  resting 
stage,  if  kept  dry,  can  endure  a  temperature  of  100°  C,  the  tem- 
perature of  boiling  water,  without  having  their  vitality  impaired, 
while  their  embryos,  if  active,  would  perish  at  60°  C. 

The  length  of  the  resting  period  varies  much  for  different  kinds 
of  seeds  and  for  seeds  of  the  same  kind.  In  a  sample  of  Clover 
seed,  for  example,  many  of  the  seeds  may  germinate  in  two  or 
three  days,  and  some  may  not  germinate  for  a  month  or  a  year. 
Although  the  seeds  of  some  wild  plants  will  germinate  as  soon  as 
mature,  if  given  favorable  conditions  of  moisture  and  warmth, 
most  of  them,  however,  have  a  rest  period  which  extends  over 
days,  weeks,  months,  or  even  years,  and  often  saves  the  young 
plants  from  getting  started  at  a  time  when  they  would  soon  be 
caught  by  unfavorable  conditions.  Excepting  some  seeds  like 
those  of  the  Clovers  and  Alfalfa,  the  seeds  of  cultivated  plants  will 
usually  germinate  about  as  soon  as  mature.  Although  a  desirable 
feature,  it  sometimes  results  in  loss,  in  that  Corn,  Wheat,  Oats, 
and  other  crops  germinate  in  the  field  if  the  weather  following 
harvest  is  warm  and  wet.  The  resting  period,  which  is  retained 
by  Wild  Oats  and  some  other  wild  plants  kindred  to  cultivated 
ones,  has  been  lost  from  our  cultivated  plants  through  many 
years  of  selection. 


VITALITY   AND   VIGOR   OF  SEEDS  69 

In  preventing  the  absorption  of  water  and  oxygen,  which  are 
the  elements  upon  which  germination  in  most  cases  depends,  the 
seed  coat  and  other  protective  structures  are  important  factors. 

Seed  coats  that  prevent  the  escape  of  water  and  thus  protect 
the  embryo  against  excessive  drying  also  prevent  the  entrance  of 
water,  and,  if  the  seed  coat  is  too  impervious  to  water  and  air, 
the  germination  of  the  seed  is  delayed.  Seeds  which  have  very 
hard  coats,  unless  they  are  treated  artificially,  must  be  exposed 
to  the  weather  until  the  seed  coat  is  decayed  sufficiently  to  allow 
the  entrance  of  water  and  air  to  the  embryo  before  germination 
can  take  place.  In  a  sample  of  Red  Clover,  Sweet  Clover,  and 
Alfalfa  seed,  often  there  are  many  seeds,  known  as  hard  seeds,  with 
coats  so  hard  that  germination  is  delayed  or  prevented.  When 
sown,  they  either  lie  in  the  ground  too  long  before  germination  or 
do  not  germinate  at  all.  By  scratching  or  pricking  their  seed  coats, 
so  that  water  and  air  can  enter  more  readily,  they  germinate  more 
promptly.  Experiments^  have  shown  that  Clover  seed  which 
has  been  thrashed  through  a  huller  where  it  is  scratched  by  the 
spikes  germinates  much  better  than  seed  hulled  by  hand.  This 
principle  is  so  well  recognized  that  machines  especially  devised 
for  scratching  or  pricking  the  coats  of  Clover  seed  have  been  in- 
vented. The  opening  of  the  seed  coats  of  the  Sweet  Pea  and 
Canna  with  a  file  and  Peach  pits  with  a  hammer  are  other  in- 
stances in  which  the  rest  period  is  broken  by  artificial  means. 

In  some  cases,  as  in  the  Hawthorne,  delayed  germination  de- 
pends upon  the  embryo,  which  must  undergo  a  process  known  as 
"  after-ripening  "  in  which  acids,  enzymes,  or  other  essential  sub- 
stances are  formed.  In  some  weed  seeds,  delayed  germination 
has  been  found  to  depend  upon  the  toughness  of  the  seed  coat, 
which  allows  water  and  air  to  enter,  but  is  so  resistant  to  pressure 
that  it  will  not  allow  the  embryo  to  expand  until  its  resistance  is 
weakened  by  decay. 

Vitality  and  Vigor  of  Seeds.  —  Seeds  are  worthless  for  planting 
unless  they  have  life,  or  vitality.  Not  only  the  vitahty,  but  also 
the  amount  of  life  or  vigor  the  seed  has  is  an  important  feature. 
If  the  embryo  of  a  seed  is  dead,  the  seed  will  not  germinate.  If 
the  embryo  is  lacking  in  energy,  though  it  may  germinate,  the 
plant  which  it  produces  will  be  weak.  Only  seeds  with  vigorous 
embryos  are  fit  for  planting. 

1  See,  Bulletin  177,  Vermont  Agricultural  Experiment  Station. 


70 


SEEDS  AND   FRUITS 


The  vitality  and  vigor  of  seeds  depend  upon  the  following 
factors:  (1)  the  vigor  of  the  plant  which  produced  the  seeds;  (2) 
external  conditions  which  affect  seeds  during  their  development; 
(3)  maturity  of  seeds;  (4)  weight  and  size  of  seeds;  (5)  methods 
of  storing;  and  (6)  age  of  seeds. 

The  seeds  of  vigorous  plants  are  preferable  to  those  of  weak 
plants,  for  the  sperms  and  eggs  of  vigorous  plants  are  likely  to  be 
more  vigorous  than  those  of  weak  plants,  and,  therefore,  more 
capable  of  producing  vigorous  embryos.  Furthermore,  seeds 
of  vigorous  plants  may  have  more  stored  food  for  the  embryo  to 
feed  upon  during  germination  and  the  seedling  stage.  Plants 
having  a  stunted  growth,  due  to  drought,  lack  of  food,  or  attacks 
of  enemies,  are  Hkely  to  produce  small  and  often  shriveled  seeds 
which  are  lacking  in  stored  food  and  usually  have  weak  embryos. 

Seeds  are  often  injured  by  frosts  occurring  while  the  seeds  are 
immature  and  full  of  water.  The  embryos  of  Corn  and  other 
seeds  are  sometimes  killed  by  early  frosts.  Even  seeds  which 
have  reached  maturity  cannot  endure  hard  freezing  unless  they 
are  dry.  For  this  reason  most  seeds  should  be  collected  from 
the  field  before  they  have  been  exposed  to  a  hard  freeze. 

Abnormal  seeds  have  a  low  vitality  or  will  not  germinate  at  all. 
Kernels  of  Corn  produced  on  the  tassel  usually  give  a  low  per- 
centage of  germination.  Sometimes,  as  in  case  of  Sweet  Clover 
and  Alfalfa,  when  the  conditions  are  unfavorable,  seeds  are  pro- 
duced with  imperfect  embryos  which  are  not  capable  of  developing 
plants.  There  are  some  plants  in  which  seeds  sometimes  develop 
without  embryos  and  of  course  will  not  germinate  at  all.  This 
sometimes  occurs  in  the  Apple  and  Pear.  When  seeds  are  muti- 
lated their  vitality  is  usually  impaired.  Larbaletrier  asserts  that 
15  per  cent  of  the  Wheat  crop  in  France  is  injured  by  the  thresh- 
ing machine.  He  cut  the  kernels  with  a  knife  so  as  to  represent 
the  injury  from  the  machine  and  compared  their  germinative 
power  with  that  of  sound  kernels,  obtaining  a  much  lower  per- 
centage of  germination  as  the  results  given  in  the  table  below 


Sound  kernels,  per 
cent  of  germination. 

Cut  kernels,  per  cent 
of  germination. 

68 

74 
99 

34 
3 

38 

LONGEVITY  71 

show.  Sturtevant  mutilated  the  kernels  of  a  Flint  Corn  and  the 
seeds  of  Beans  and  found  the  percentage  of  germination  nmch 
reduced  in  each  case. 

Seeds  collected  while  immature  usually  show  a  low  percentage 
of  germination  and  their  embryos  grow  slowly.  In  the  case  of 
Rye,  seeds  have  been  harvested  at  different  stages  of  their  devel- 
opment and,  after  similar  treatment  in  respect  to  drying  and 
storage,  the  percentage  of  germination  and  vigor  of  embryos  de- 
termined. In  the  milk  stage  five  per  cent  germinated,  while  in 
the  dry  ripe  stage  eighty-four  per  cent  germinated.  The  embryos 
of  the  dry  ripe  seeds  were  much  more  vigorous  in  growth  than 
those  of  the  immature  seeds.  Tomato  seeds,  while  still  green  and 
not  more  than  two-thirds  the  weight  of  mature  seeds,  may  be 
germinated,  if  properly  cured,  but  the  plants  produced  are  likely 
to  be  weak.  The  germination  of  unripe  seeds  has  been  given 
considerable  attention  by  Sturtevant,  Arthur,  and  Golf.^ 

Experiments-  with  seeds  of  the  Radish,  Sweet  Pea,  Cane,  Rye, 
Oats,  and  Cotton  have  shown  that  better  stands  in  the  field  and 
more  vigorous  and  better  yielding  plants  are  secured  by  using 
only  the  heavier  seeds. 

The  vitality  and  vigor  of  seeds  depend  very  much  upon  the 
methods  of  storing.  Seeds  are  more  easily  killed  by  extremes  of 
temperature  when  wet.  Seeds  stored  where  there  is  considerable 
moisture  may  start  to  germinate,  and  then  die.  Seeds,  massed 
together  before  they  are  well  dried,  become  moist  and  often  so 
warm  that  the  embryos  are  injured.  On  the  other  hand,  when 
stored  in  rooms  where  the  air  is  warm  and  extremely  dry,  seeds 
may  lose  moisture  so  rapidly  that  the  embryos  are  killed.  A 
storage  room  should  be  cool  Init  above  freezing,  and  dry,  although 
not  excessively  dry.  Until  the  seeds  are  well  dried,  they  should 
not  be  massed  together,  but  so  arranged  that  the  air  can  circulate 
about  them.  Thus  methods  of  storing  seed  Corn  and  other  seeds 
must  reckon  with  a  number  of  factors  which  affect  the  vitality 
and  vigor  of  seeds  under  storage  conditions. 

Longevity.  —  The  vitality  and  vigor  of  seeds  depend  nmch 
upon  their  age.  Seeds  in  excellent  condition  and  stored  by  the  best 
methods  finally  lose  their  vitality,  due  to  the  coagulation  of  their 
protoplasms,  too  much  drying,  or  some  other  factor  not  under- 

*  American  Naturalist,  pp.  806  and  904.     1895. 
2  Farmers'  Bulletin  676,  U.  S.  Dept.  of  Agriculture. 


72  SEEDS  AND    FRUITS 

stood.  Some  seeds  may  retain  theii-  vitality  for  centuries,  but 
most  seeds  lose  it  in  a  few  years.  The  length  of  time  during  which 
seeds  retain  their  vitahty  is  called  their  longevity.  Most  agri- 
cultural seeds  can  be  stored  two  or  three  years  without  much  loss 
of  vitalit}',  and  some,  when  stored  a  nmch  longer  period,  may 
contain  a  large  number  of  live  seeds.  One  investigator  found 
that  50  per  cent  of  samples  of  Red  Clover  seeds  germinated  after 
being  stored  in  bottles  for  12  years;  and  in  samples  of  the  seeds 
of  Pigweed,  Sheep  Sorrel,  Black  Mustard,  and  Pepper  Grass, 
stored  in  the  same  way,  a  large  percentage  germinated  after  a 
storage  of  25  yeais.  In  samples  of  White  Sweet  Clover  seeds, 
which  have  well  mod  tied  seed  coats,  18  per  cent  have  germinated 
after  a  storage  of  50  years.  There  is  good  evidence  that  some 
of  the  leguminous  seeds  may  retain  their  vitality  for  more  than 
a  century.  Many  of  the  weed  seeds  when  buried  in  the  soil  can 
retain  their  vitality  for  many  years  and  then  germinate  when 
conditions  become  favorable. 

The  longevity  of  seeds  depends  so  much  upon  the  conditions 
under  which  the  seeds  were  grown,  maturity  when  collected,  and 
methods  of  storing,  that  statements  as  to  how  old  any  kind  of 
seeds  may  be  and  still  be  safe  for  planting  are  not  reliable.  Old 
seeds  are  often  preferable  to  new  ones  grown  under  unfavorable 
conditions.  Seeds  from  poorly  developed  plants,  although  sim- 
ilar in  appearance  to  those  produced  under  favorable  conditions 
and  giving  a  high  percentage  of  germination  soon  after  harvest, 
decline  rapidly  in  vitality,  often  being  worthless  at  the  next  plant- 
ing season.  For  example.  Cabbage  seeds  eight  years  old  may 
germinate  70  or  80  per  cent,  while  some  only  three  years  of  age 
but  grown  in  an  unfavorable  year  may  germinate  less  than  40 
per  cent.  Seeds  collected  green  may  germinate  well  after  proper 
curing  but  they  have  a  short  longevity. 

The  longevity  of  seeds  depends  probably  more  upon  dryness 
than  any  other  factor.  For  this  reason  the  place  of  storage 
should  be  dry  and  the  seeds  should  be  cured  before  they  are 
stored  by  placing  them  in  a  dry  airy  place.  Experiments  show 
that  Corn  collected  soon  after  maturity  and  properly  cured  and 
stored  gives  a  much  higher  percentage  of  germination  the  next 
season  than  Corn  allowed  to  stand  in  the  shock,  or  taken  from  the 
crib.  Comparative  '  germinative  tests  of  seeds  stored  in  different 
1  Bulletin  58,  Bureau  of  Plant  Industry,  U.  S.  Dept.  of  Agriculture. 


LONGEVITY 


73 


parts  of  the  United  States  have  shown  that  seeds  do  not  hve  as 
long  in  the  warm  moist  air  of  the  Southern  states  as  they  do  in 
the  cool  dry  air  of  the  Northern  states. 

In  the  following  table  compiled  from  various  sources  is  given 
the  time  beyond  which  it  is  not  advisable  to  use  the  seeds  men- 
tioned unless  the  contrary  is  shown  by  germinative  tests. 


Corn 

Wheat 

Oats 

Barley 

Rye 

Buckwheat 

Beans  (common) 4  to  5 

Peas 4  to  5 

Clovers 2  to  3 

AKaKa 3  to  4 

Onion 1 


Years. 
2 
2 

2 
2 

2 
2 


Years. 
Mustard 3  to  4 


Cabbage 

Turnips 

Swede 

Pumpkin 

Melon  (musk) . 
Melon  (water) 

Squash 

Tomato 

Timothy 

Celery 


3  to  4 

3  to  4 

3  to  4 

5 

5 

5 

3 

6 

lto2 

1 


In  some  cases  perfect  seeds  well  stored  may  have  more  than 
double  the  longevity  given  in  the  above  table.     Thus  Sturtevant 
obtained  100  per  cent  germination  of  various  varieties  of  Corn 
after  being  stored  5  years.    Tomato  seeds 
14  years  old  have  been  known  to  give  a 
high  percentage  of  germination.     On  the 
other  hand,  using  the  same  seeds  as  an 
example,  both  Corn  and  Tomato  seeds 
are  sometimes  unfit  for  use  when  only 
1  year  of  age.    These  varying  results  em- 
phasize   the    importance   of    testing  the 
germinative  power  of  seeds  before  use. 

The  variation  in  the  longevity  of  the 
seeds  of  a  given  lot  is  obvious  when  the 
percentages  of  germination  for  different 
periods  of  storage  are  compared.  The 
decrease  in  the  percentage  of  germination 
as  the  length  of  the  storage  period  in- 
creases shows  that  some  seeds  die  earl;y 
and  others  later  until  finally  all  are  dead. 

In  the  following  table  are  given  the  results  of  an  experiment 
to  determine  the  rate  at  which  vitality  is  lost  as  indicated  by  the 
percentage  of  germination  obtained  in  each  of  the  6  years  of 
storage. 


Fig.  72. — ^A  cheap  mag- 
nifier well  adapted  for  use 
in  analyzing  seeds.  The 
magnifier  is  set  over  the 
seeds,  leaving  the  hands 
free  to  separate  the  seeds 
as  one  looks  through  the 
magnifier  from  above. 


74 


SEEDS  AND   FRUITS 


PER  CENT  OF  GERMINATION   FOR   EACH  OF  6  YEARS 
OF  STORAGE 


Wheat 
Oats . . 
Barley 
Peas . . 

Flax.. 


1  yr. 

2yr. 

3  yr. 

4yr. 

5yr. 

80 

82.3 

77.3 

37 

15 

90.2 

93 

78.2 

67 

54 

97 

91 

78.5 

36 

19.5 

94 

95 

88 

64 

64 

81 

82 

75 

49 

26 

29.5 
7.5 


24 


Purity  and  the  Analysis  of  Seeds 

The  impurities  of  seeds  consist  of  seeds  of  other  species  and  of 
dirt,  such  as  soil  particles,  chaff,  hulls,  and  other  plant  fragments. 
In  sowing  impure  seeds  one  can  not  estimate  the  amount  of 
desirable  seeds  sown  unless  the  percentage  of  impurities  is  pre- 
viously determined  so  that  allowance  can  be  made.  Besides  one 
is  likely  to  sow  the  seeds  of  undesirable  plants,  which  choke  the 
crop  and  cause  much  trouble  and  expense  in  eradicating  them. 
A  small  per  cent  of  weed  seeds  is  often  a  serious  matter.  For 
example,  in  sowing  Grass  seeds  which  contain  only  1  per  cent  of 
weed  seeds  there  is  the  possibility  of  20  or  more  weeds  to  the 
square  yard.  Nobbe  found  enough  weed  seeds  in  a  certain 
sample  of  Timothy  seeds,  if  sown  at  the  ordinary  rate,  to  supply 
24  weeds  to  every  square  foot  of  land.  Furthermore,  in  purchas- 
ing impure  seeds,  unless  a  deduction  from  the  price  is  made  for 
the  impurities,  one  pays  more  than  he  should  for  the  desirable 
seeds  obtained. 

More  impurities  occur  among  the  smaller  agricultural  seeds, 
as  Grass,  Clover,  and  Alfalfa  seeds,  than  among  the  grains,  al- 
though a  few  very  bad  weed  seeds,  such  as  those  of  Quack  Grass 
(Agropyron  re-pens),  Cow  Cockle  {Saponaria  Vaccaria),  Corn 
Cockle  {Lychnis  Githago),  and  English  Charlock  (Brassica 
Sinapistrum) ,  are  common  among  the  small  grains. 

Seed  Analysis.  —  A  bag  of  seeds  may  be  analyzed  for  two 
reasons:  (1)  to  determine  the  percentage  of  the  desirable  seeds 
contained  or  to  determine  the  percentage  of  impurities  regardless 
of  their  kinds;  and  (2)  to  determine  the  kinds  of  impurities  and 
the  percentage  of  each  present.  In  either  case  the  determination 
is  based  upon  the  analysis  of  only  a  small  sample,  which  is  usually 
prepared  by  mixing  well  a  handful  or  more  of  seeds  taken  from 


SEED   ANALYSIS 


75 


different  parts  of  the  bag  or  container,  usually  from  the  top, 
middle,  and  bottom.  From  the  sample  from  2  to  5  grams  are 
weighed  out,  and  the  impurities  and  desirable  seeds  are  then  sepa- 
rated, usually  by  means  of  a  lens  like  the  one  in  Figure  12.     By 

e.^fe  09^  0% 

Tied  clove*-  TvefoW  Cuv\e(i(iock 

^#  V  ^@ 

-     4  ,       ,^  Lamb's        6V\l\\a        ^ 

Laciv^^tUvvnA*         q.ua^'Vers         mustard 


^ 


#?i 


9Tv.e\d, 
doddc 


ioM-J.r^ 


'j>\unVa 


8CVpve\:     , 
cXodaev 

^.0 


a?/ 


'a^Qy 


^  ^^   14Canad:c.     15WAA   ^ 

Fig.  73.  —  Some  weed  seeds  and  fruits  commonly  found  among  Red  Clover 
seeds.  Enlarged  and  about  natural  size.  From  Farmers'  Bulletin  4^5, 
U.  S.  Dept.  of  Agriculture. 


dividing  the  weight  of  the  desirable  seeds  and  the  weight  of  the 
impurities  by  the  number  of  grams  analyzed,  the  percentage  of 
each  is  obtained.     Thus,  if  5  grams  are  analyzed  and  the  weight 

4.8 


of  the  desirable  seeds  found  is  4.8  grams,  then 


96  per  cent, 


which  is  the  percentage  of  purity.     In  determining  the  kinds  of 


76 


SEEDS  AND   FRUITS 


impurities  and  their  percentages  it  is  not  enough  to  separate  the 
impurities  and  desirable  seeds,  but  the  kinds  of  impurities  must 
be  identified,  separated,  and  the  weight  necessary  for  finding  the 
percentage  of  each  nnist  l)e  separate!}^  determined.     In  this  kind 


10  lamb's  quarters      H  Spreading 


'Creen  fii>rt3i/ 


Fig.  74.  —  Some  weed  seeds  commonly  found  among  Alfalfa  seeds.  En- 
larged and  natural  size.  Adapted  from  Farmers'  Bulletin  495,  U.  S.  Dept. 
of  Agriculture. 

of  analysis,  the  operator,  unless  he  is  well  acquainted  with  the 
various  kinds  of  seeds,  should  have  at  hand  for  comparison 
samples  or  figures  of  the  seeds  of  weeds  and  other  plants  likely 
to  occur  among  the  seeds  which  are  being  analyzed.  Samples 
are  better,  but  figures  as  shown  in  Figures  73  and  74  may  serve 
quite  well. 


TOMATO   OR   BERRY   TYPE 


77 


Nature  and  Types  of  Fruits  of  Flowering  Plants 

A  fruit  is  difficult  to  define  because  not  all  fruits  involve  the 
same  structures  in  their  formation.  Some  fruits  are  only  much 
enlarged  ovaries;  but  there  are  others  which  involve  other  struc- 


FiG.  75.  —  A,  cross  section  of  a  Tomato.     B,  cross  section  of  an  Orange. 
w,  ovary  wall;   p,  placentas;   s,  seeds;   a,  partition  walls;   I,  locales. 

tures  closely  related  to  the  ovary.  Since  fruits  involve  a  number 
of  structures  in  their  formation,  it  will  be  best  to  study  some 
types  and  then  formulate  a  definition. 

Tomato  or  Berry  Type.  —  The  fruit  of 
the  Tomato  consists  of  the  ovary  which 
has  enlarged  and  become  fleshy  and  juicy. 
The  most  edible  portion  consists  of  the 
fleshy  enlargements  which  develop  from 
the  inner  angle  of  the  locules  and  almost 
fill  them.  These  enlargements  bear  th(^ 
seeds  and  hence  are  the  placentas  much 
enlarged.  Also  the  citrus  fruits,  such  as 
Oranges,  Lemons,  etc.,  are  of  the  berry 
type.  However,  they  have  no  fleshy 
placentas.  The  seeds  are  attached  to  the 
small  central  core,  and  the  juicy  tissues 
developing  from  other  parts  of  the  ovary 
and  fining  the  locules  constitute  the  flesh 
of  these  fruits.  The  fleshy  and  juicy 
features  are  characteristics  of  the  berry; 
defined  as  a  fleshened  juicy  ovary.     {Fig 


Fi(J.  76.  —  Lengthwise 
section  through  a  Plum. 
s,  seed;  p,  wall  of  pit; 
/,  fleshy  portion  of  ovary. 

and  a  berry  is  often 

.  75.) 


78 


SEEDS  AND   FRUITS 


Plum  or  Stone  Type.  —  The  Plum,  Peach,  Cherry,  and  Apri- 
cot, commonly  called  drupes,  are  fleshy  ovaries,  but  differ  from 


Fig.  77.  —  Section  through  flower  and  fruit  of  the  Apple.  A,  section 
through  the  flower,  a,  receptacle;  b,  ovaries;  d,  ovules;  /,  floral  organs, 
calyx,  corolla,  stamens,  styles  and  stigmas.  B,  section  through  the  fruit. 
a,  receptacle;  c,  core;  s,  seeds;  r,  remains  of  floral  parts;  /,  the  flesh  around 
the  core,  bounded  on  the  outside  by  the  conductive  vessels,  indicated  by  the 
Unes.  The  inner  portion  of  this  band  of  flesh  is  the  outer  portion  of  the  ovaries, 
the  remainder  of  it  being  the  inner  portion  of  the  receptacle. 

the  berry  type  in  that  the  portion  of  the  ovary  immediately  sur- 
rounding the  locule  hardens  into  the  stone  or  pit.  In  Figure  76, 
point  out  the  seed,  the  pit,  and  the  fleshy 
portion  of  the  ovary. 

Apple  or  Pome  Type.  —  The  Apple, 
Pear,  and  Quince  are  examples  of  pome 
fruits,  and  their  structure  can  best  be  un- 
derstood by  studying  Figure  77.  The 
receptacle  of  the  flower  is  not  flat  as  it 
is  in  many  flowers,  but  is  hollow  or  urn- 
shaped;  and  the  five  ovaries  are  located 
in  the  hollow  of  the  receptacle  and  are 
of  a  Cucumber,  r,  rind  grown  fast  to  its  sides.  The  calyx,  petals, 
consisting  of  receptacle  and  stamens  are  located  on  the  rim  of 
and  ovary  waU  closely  the  receptacle  and  thus  above  the  ova- 
joined;  I,  locules;  p,  pla-    ^.j^^^    ^^  ^^^  f^.^-^  develops,  the  receptacle 

cpntSiS'  s   soocis 

'    '        "  surrounding    the    ovaries    thickens    and 

forms  the  greater  part  of  the  fruit,  while  the  ovaries  form  the 
portion  known  as  the  core. 


Fig.  78.  —  Cross  section 


BLACKBERRY  T\PE 


79 


Melon  or  Pepo  Type.  — •  In  the  Melons,  Cucumbers,  Pump- 
kins, and  Squashes,  which  ilhistrate  well  the  pepo  type,  the 
ovaries  are  inclosed  in  the  receptacle,  and  with  the  receptacle  to 


Fig.  79.  —  Flower  and  fruit  of  Strawberry.  A,  section  through  flower, 
showing  the  fleshy  receptacle  (jr)  and  the  many  pistils  {p)  on  its  surface. 
B,  fruit  consisting  of  enlarged  receptacle  (r),  bearing  the  small  hard  ovaries  (o). 


which  they  are  closely  joined  form  the  rind.  (Fig.  78.)  The 
placentas  are  more  or  less  fleshy  and  in  case  of  the  Watermelon, 
where  they  form  large  juicy  lobes,  they  constitute  the  bulk  of 
the  edible  portion.  In  most  cases,  however,  as  Muskmelons  and 
Pumpkins  illustrate,  the  placentas  break 
loose  from  the  ovary  wall  and  are  removed 
with  the  seeds.  In  what  way  does  the 
Melon  resemble  the  Apple  in  structure? 
How  does  it  differ  from  the  Apple? 

Strawberry  Type.  —  In  the  Strawberry 
the  ovaries  develop  into  hard  one-seeded 
fruits  (akenes)  which  appear  as  small  hard 
bodies  over  the  surface  of  the  much  flesh- 
ened  receptacle.  {Fig.  79.)  In  the  Straw- 
berry, although  the  ovaries  are  included 
when  the  fruit  is  used,  the  edible  portion 
is  the  receptacle. 

Blackberry  Type.  —  In  this  type  the 
ovaries  develop  as  small  stone  fruits,  often 
called  drupelets  (miniature  drupes),  and 
with  the  fleshened  receptacle  form  the 
fruit.  {Fig.  80.)  Very  similar  to  the  Blackberry  is  the  Rasp- 
berry, in  which  the  drupelets  collectively  separate  from  the  re- 
ceptacle and  thus  alone  form  the  fruit. 


Fig.  80.  — Fruit  of  the 
Blackberry,  r,  recepta- 
cle; /,  fleshened  ovaries. 


80 


SEEDS   AND   FRUITS 


In  developing  from  a  single  flower  ])ut  involving  a  number  of 
pistils,  the  fruits  of  the  Strawberry  and  Blackberry  are  similar 
and  are  classed  as  aggregate  fruits. 

Pineapple  Type.  —  In  the  formation  of  the  Pineapple  a  num- 
ber of  flowers  are  involved,  each  of  which  consists  of  a  small 
pistil  surrounded  by  large  scales  and  is  borne  in  the  axil  of  a  modi- 
fied leaf.  Each  ovary  with 
its  scales  and  modified  leaf 
becomes  fleshy  to  form  a 
single  fruit.  The  entire  fruit 
of  the  Pineapple  consists  of  a 
number  of  these  single  fruits 
closely  packed  together  on 
an  axis  which  forms  the  core 
of  the  Pineapple.  Since  a 
number  of  flowers  are  in- 
volved, fruits  of  this  type 
are  known  as  multiple  fruits. 
{Fig.  SI.) 

Nut  Type.  — ^  In  the  nut 
type  of  fruit,  the  ovary  is 
hard  and  is  generally  partly 
or    entirely    covered    by    a 


husk  formed  by  the  perianth 
or  by  bracts  which  grow  up 
from  the  receptacle.  {Fig. 
82.)  Notice  the  develop- 
ment of  the  Acorn  shown  in 
Figure  83. 

Some  Other  Familiar  Types  of  Fruits.  —  In  many  small  fruits 
the  ovaries  become  dry  and  often  hard  as  the  fruit  matures. 
They  are  the  kind  which  when  small  and  one-seeded  are  often 
called  seeds.  It  has  been  mentioned  that  the  akenes  of  the  Buck- 
wheat and  the  cariopsis  of  the  Grasses  are  fruits  with  hard  ovary 
walls.  In  the  Clovers,  Alfalfa,  and  Beans  the  ovary  wall  becomes 
dry  and  hard  when  mature,  forming  the  structure  known  as  the 
pod  or  legume.  {Fig.  84-)  Many  of  the  so-called  weed  seeds  are 
dry  ovaries.  In  many  cases,  however,  other  structures  are  joined 
with  the  hardened  ovary  in  the  formation  of  the  fruit.  In  the 
Dandelion  and  many  other  plants  of  the  Composite  type,  the 


Fig.  81.  —  Pineapple.     After  Koch. 


DEFINlTIOxN   OF   A   FRUIT 


81 


pappus,  consisting  of  hair-like  structures  which  correspond  to 
the  calyx  of  the  ordinary  type  of  flower,  remains  as  a  part  of  the 
fruit,  forming  a  parachute-like  arrangement  which  enables  the 


Fig.  82.  —  Pistillate  flower  and  fruit  of  a  Hickory  (Carya).  A  and  B,  ex- 
terior and  interior  views  of  the  flower.  C,  the  nut.  b,  bracts  surrounding  the 
pistil  (p);  0,  ovary.     Flower  much  enlarged  but  fruit  reduced. 

fruit  to  float  in  the  air.  Sometimes,  as  in  the  Spanish  Needles, 
the  calyx  remains  on  the  fruit  as  spiny  appendages.  In  the  case 
of  the  Birch,  Elm,  Ash,  and  Maple,  the  fruit  known  as  a  samara 
or  key-fruit  has  wing-like  structures  which  are  outgrowths  from 
the  ovary  wall. 


Fig.  83.  —  Flower  and  fruit  of  an  Oak  (Quercus).  A,  pistillate  flower, 
showing  the  bracts  (6)  which  surround  the  ovary.  B,  section  of  the  flower, 
showing  the  ovary  (o)  and  the  bracts  (6).  C,  acorn,  showing  the  ovary  and 
cup.     s,  stigmas.     Flower  much  enlarged  but  fruit  nearly  natural  size. 


Definition  of  a  Fruit.  —  From  an  examination  of  the  above 
types  of  fruits,  it  follows  that  a  fruit  may  consist  of:  (1)  simply 
the  ovary  either  dry  or  fleshy;  (2)  ovary  or  ovaries  and  recep- 


82 


SEEDS  AND   FRUITS 


taclo;  (3)  ovary  with  perianth  or  l)racts  forming  a  husk;  (4)  ovary 
with  calyx  foniiinu;  hairs  or  spines;  and  (5)  a  number  of  single 
fruits  with  the  modified  leaves  and  floral 
axis  of  the  flower  group.  A  fruit  may 
be  defined  as  one  or  more  ripened  ovaries 
either  with  or  without  closely  related 
parts. 

Dissemination  of  Seeds  and  Fruits 

Dissemination  has  to  do  with  the 
scattering  of  seeds  from  the  parent  plant. 
Sometimes  the  seed  is  transported  naked, 
but  often  it  is  transported  enclosed  in 
the  fruit  or  with  some  larger  part  of  the 
plant. 

The  necessity  for  dissemination  is  ob- 
vious, for  if  the  seeds  of  a  plant  were  to 

jPj^  g4 rpj^^^  ,     coiled  germinate  where  formed  or  on  the  ground 

directly  beneath,  the  resultant  conges- 
tion would  prevent  the  normal  develop- 
ment of  any  of  the  plants.  Green 
plants  must  have  sunlight  and  air,  and 
this  means  that  they  must  have  room. 

Of  course  seeds  and  fruits  are  not  the  only  means  by  which 
plants  spread.  Many  Seed  Plants  have  an  additional  means  in 
either  spreading  stems  or  roots  which  give  rise  to  new  plants  as 
they  spread  farther  and  farther  from  the  parent.  The  Straw- 
berry depends  mainly  upon  its  runners,  and  the  Quack  Grass 
much  upon  its  underground  stem  as  a  means  of  spreading.  Pop- 
lars, some  fruit  trees,  and  Canada  Thistle  are  well  known  to 
spread  by  means  of  sprouts  arising  from  their  roots.  Most  plants 
which  do  not  have  seeds  spread  by  means  of  spores  which  in  some 
cases  seem  to  be  a  more  efficient  means  than  seeds  are.  For 
example,  Wheat  Rust,  a  disease  which  spreads  very  rapidly,  is 
spread  by  spores. 

In  the  dissemination  of  seeds  and  fruits,  wind,  water,  and  ani- 
mals are  the  chief  agents.  In  a  few  plants  there  are  explosive  or 
spring-like  mechanisms  which  throw  the  seeds. 

Seeds  and  Fruits  Carried  by  Wind.  —  The  wind  is  one  of  the 
most  important  agents  in  the  distril)ution  of  fruits  and  seeds      In 


fruits  (pods)  of  Alfalfa 
{Medieago  sativa).  From 
Farmers'  Bulletin  895,  U.  S. 
Dept.  of  Agriculture. 


SEEDS  AND   FRUITS   CARRIED   BY   WIND 


83 


the  Thistle,  Dandelion,  Wild  Lettuce,  Fiieweed,  Ironweed,  White 
Weed,  Fleabane,  and  others,  the  tufts  of  downy  hairs  on  the  small 
dry  fruits  in  which  the  seeds  are  enclosed  enable  the  fruits  with  the 
seeds  to  be  lifted  and  carried  many  miles  by  the  wind.  In  the 
Milkweeds,  the  seeds  bear  long  hairs  which  make  them  easily 
carried  by  the  wind.  In  some  plants,  as  in  the  Curled  and  Smooth 
Dock,  Ash,  Elm,  and  Maple,  the  fruits  are  winged  and  easily 
borne  away  by  a  passing  breeze.     The  fruits  of  some  of  the 


Fig.  85.  —  Some  fruits  and  seeds  disseminated  by  the  wind,  a,  fruits  of  the 
Basswood  (T ilia  Americana)  and  the  leaf-like  bract  which  floats  in  the  air  and 
thereby  scatters  the  fruits,  h,  samara  or  winged  fruit  of  a  Maple,  c,  fruit  of 
a  Wild  Lettuce  {Lactuca  Floridana).  d,  winged  fruit  of  an  Elm.  e,  pods  of  a 
Milkweed  {Asclepias  syriaca)  allowing  the  seeds  to  escape  to  be  scattered  by 
the  wind,     a,  c,  and  e  from  Hayden. 


Grasses  are  enclosed  in  chaff  bearing  long  hairs  and  are  easily 
blown  about.  The  fruits  and  seeds  of  Ragweeds,  Velvet-leaf, 
Docks,  Pigweeds,  Chickweeds,  and  some  plants  of  the  Grass  fam- 
ily are  blown  long  distances  over  the  surface  of  snow,  ice,  or 
frozen  ground.     {Fig.  85.) 

Some  plants  break  off  near  the  ground  after  ripening  their  seeds 
and  are  rolled  over  and  over  by  the  wind,  dropping  their  seeds  as 
they  go.  These  are  known  as  the  "tumble-weeds"  and  include 
the  Russian  Thistle,  Tumbling  Mustard,  Tumbling  Pigweed, 
Buffalo  Bur,  Old  Witch  Grass,  and  a  number  of  others.    {Fig.  S6.) 


84 


SEEDS  AND   FRUITS 


Seeds  and  Fruits  Carried  by  Water.  —  Plants,  such  as  the 
Great  Ragweed,  Sniartweeds,  Bindweeds,  Willows,  Poplars,  and 
Walnuts,  which  grow  along  streamy  have  their  seeds  and  fruits 
floated  away  during  overflows.     Sometimes,  when  the  banks  of 


Fig.  86.  —  Plants  of  the  tumble  weed  {Amaranlhiis  albus)  tumbling  over 
the  ground  and  scattering  seeds  as  they  go.     After  Bergen. 

streams  cave  off,  plants  with  ripened  seeds  fall  into  the  current 
bodily  and  are  carried  for  miles  down  the  stream,  finally  lodging 
in  fields  where  their  seeds  grow.  The  seeds  of  plants  growing  on 
the  upland  are  washed  to  the  lowlands  during  rains  and  seed  the 
bottom  fields.  Some  fruits,  as  in  case  of  the  Coconut,  are  so 
resistant  to  salt  water  that  they  can  be  carried  long  distances  by 
ocean  currents. 

Seeds  and  Fruits  Carried  by  Animals.  — •  Birds  eat  the  fruits 
of  some  plants  for  the  outer  pulp,  and  the  hard  seeds  pass  undi- 
gested. In  this  way  the  seeds  of  the  Nightshades,  Poison  Ivy, 
Pokeweed,  Blackberry,  Pepper  Grass,  and  others  are  distributed. 
Even  the  seeds  and  fruits  of  Thistles,  Dandelion,  Ragweeds,  and 
Knotgrass  may  be  eaten  in  such  large  quantities  that  many  pass 
undigested  and  start  new  plants  wherever  they  fall.  Birds  often 
carry  sprigs  of  plants  to  places  where  the  seeds  may  be  eaten 
without  molestation  and  in  this  way  distribute  seeds.  {Fig.  87.) 
Birds  that  wade  in  the  edge  of  ponds,  lakes,  and  streams 
often  carry  away  on  their  feet  and  legs  mud  containing  seeds. 


SEEDS  AND   FRUITS  CARRIED   BY  ANIMALS 


85 


Darwin  took  3  tablespoonfuls  of  mud  from  beneath  the  water  at 
the  edge  of  a  pond  and  kept  it  in  his  study  until  the  seeds  con- 
tained developed  into  plants.  From  this  small  amount  of  mud, 
he  obtained  537  plants  which  represented  a  number  of  species. 
From  this  it  is  evident  that  the 
mud,  carried  on  the  feet  and  legs 
of  water  birds,  may  be  the  means 
of  distributing  many  seeds. 

The  fruits  and  seeds  of  many 
plants  have  spines  or  small  hooks 
by  which  they  become  attached 
to  passing  animals  and  are  carried 
far  and  wide.  Some  familiar  ex- 
amples are  the  burs  of  Burdock, 
Cockle  Bur,  and  Sand  Bur,  and 
the  hooked  and  spiny  fruits  of  the 
Buttercups,  Wild  Carrot,  Beggar's 
Lice,  Tick-trefoils,  Beggar-ticks, 
and  Spanish  Needles.  They  catch 
in  the  wool,  manes,  an( 

stock  and  in  the  clothing  of  man,  logical  Survey, 
and  are  carried  from  one  pasture 

to  another  or  from  one  farm  to  another.  Live  stock  are  impor- 
tant agents  in  distributing  plants  on  the  farm.  The  seeds  of 
the  Mustards  are  mucilaginous  when  wet  and,  by  sticking  to  the 
feet  of  animals  or  the  shoes  of  man,  are  carried  to  new  situations. 
{Fig.  8S.) 

Many  plants  owe  their  distribution  to  man  more  than  to  any 
other  agent.  The  railways,  connecting  all  of  the  states  and 
reaching  from  ocean  to  ocean  where  they  connect  with  steamship 
lines  from  across  the  seas,  are  responsible  for  the  wide  distribution 
of  many  plants.  For  example,  the  seeds  of  a  number  of  weeds 
are  shipped  across  the  country  with  grain  and  other  farm  seeds, 
and  also  in  hay,  bedding,  packing,  in  shipments  of  fruit,  and 
in  the  coats  of  live  stock.  They  fall  from  the  cars  as  the  train 
travels,  and  seed  the  right-of-way  where  the  plants  first  appear 
and  then  later  spread  to  the  surrounding  fields.  The  railways 
are  responsible  for  the  wide  distriljution  of  Russian  Thistle, 
Prickly  Lettuce,  Canada  Thistle,  and  Texas  Nettle,  which  first 
appear  along  the  railway  and  later  spread  to  the  surrounding 


Fig.  87.  —  A  Chickadee  carrying 
id  tails  of   fj.^i^      Yrom  Bulletin  ^,  Iowa  Geo- 


86 


SEEDS  AND   FRUITS 


farms.  Buckliorn,  Ox-eye  Daisy,  and  many  other  weeds  are 
often  first  found  along  the  railway.  Seeds  of  various  kinds  are 
often  carried  in  the  packing  around  nursery  stock.  Quack  Grass 
Canada  Thistle,  Ox-eye  Daisy,  and  other  weeds  are  often  spread 


Fig.  88.  —  Some  spiny  weed  fruits  which  catch  to  the  coats  of  animals. 
a,  cow  with  tail  loaded  with  weed  fruits,  h,  fruits  of  Beggar-ticks  (Bidens). 
c,  spiny  fruit  of  Burdock  (Arctium  Lappa),  d,  fruit  of  Comfrey  {Symphytum), 
c,  fruit  of  another  Beggar-tick.    Adapted  from  Bailey  and  from  Hayden. 


in  this  way.  Quack  Grass  is  often  carried  in  straw,  and  may 
be  introduced  on  a  farm  by  using  straw  for  covering  Grapes  and 
Strawberries.  Manure  hauled  from  livery  stables  is  a  very  im- 
portant means  of  introducing  plants  on  the  farms  where  the 
manure  is  used.  In  hauling  hay  along  the  highways,  seeds  of 
various  kinds  are  dropped  and  from  the  highways  the  plants 
spread  to  the  fields.  Those  weeds,  such  as  Quack  Grass,  White 
Top,  Field  Sorrel,  and  others  which  are  common  in  meadows, 
are  often  spread  in  this  way.     When  the  fields  are  wet. 


SEEDS   SCATTERED   BY   EXPLOSIVE   MECHANISMS 


87 


Fig.  89.  —  The   three- valved  pod  of  the 


collect  on  the  wagon  wheels  and  are  carried  to  the  highways  or 
to  other  fields.  Threshing  machines  are  important  agents  in 
scattering  seeds,  for  in  their  traveling  through  the  country  seeds 
of  various  kinds  are  jostled 
from  them  and  seed  the 
fields  and  highways. 

Man  scatters  many 
weeds  by  sowing  unclean 
seed.  Clover  seed,  Alfalfa 
seed,  Grass  seed,  Wheat, 
Oats,  etc.,  are  often  ob- 
tained from  distant  states 
or  even  from  foreign  coun- 
tries for  seeding.  Weed 
seeds  are  usually  present 
in  agricultural  seeds,  and    Violet  throwing  its  seeds.     Much  enlarged. 

,  •  ,,  After  Beal. 

sometnnes  they  are  pres- 
ent in  large  quantities.      In  tracing  weeds,  it  has  been  found 
that  many  of  the  most  troublesome  ones  have  come  from  Europe, 
Asia,  or  some  other  foreign  country.     Man  has  carried  the  seeds 
and  fruits  of  these  weeds  across  the  seas,  and  most  of  them  have 

been  imported  and  sown  with  agri- 
cultural seeds. 

Seeds  Scattered  by  Explosive 
or  Spring-like  Mechanisms.  —  In 
this  kind  of  dissemination  the 
plant  itself  is  the  agent  which, 
either  by  sudden  ruptures  due  to 
strains  or  by  explosions  due  to 
the  swelling  of  certain  tissues,  is 
^      '^     able  to  throw  the  seeds  often  a 

"^yj       'X        (^nnsirlp»"iihlp       rlistnnfp  In      the 

as   in   the 
Castor 

Fig.  90. —  The  Squirting  Cu-  Bean,  and  Field  Sorrel,  bands  of 
cumber  {Ecbalium  Elaterium)  ^igg^g  ^^.j^ich  ripen  under  tension, 
squirting  its  seeds  from  the  pod.     ^^^^  ' ^^^^  ^  ^^^^^^^  ^^^^  ^^^  p^^^ 

suddenly  rupture  with  so  much  violence  that  the  seeds  are  thrown 
in  every  direction.  In  the  Violets  the  carpels,  as  they  ripen  and 
dry,  press  harder  and  harder  upon  the  seeds,  which  suddenly 


C\^  A)vv  l\)r.v    C    ''^    ^^^^  ^°  throw  the  seeds 

r^V-^yl      considerable     distance. 
■^    pods  of   some   plants,  as 
Vetches,  Witch-hazel, 


88  SEEDS  AND   FRUITS 

shoot  out  as  a  Watermelon  seed  may  shoot  out  from  between 
one's  pressed  fingers.  (Fig.  89.)  In  case  of  the  Impatiens  called 
"Touch-me-not"  and  the  "Squirting  Cucumber/'  tissues  within 
the  pod  take  up  water  and  swell  so  much  that  the  pod  finally 
explodes  and  scatters  the  seeds  as  shown  in  Figure  90. 


cV^(3 


^ 


1^ 


h.^ 


CHAPTER  VI 

GERMINATION   OF   SEEDS:    SEEDLINGS 

Nature  of  Germination  and  Factors  upon  which  it  Depends 

Although  the  resting  condition  is  very  essential  to  the  preser- 
vation of  the  life  of  the  seed  during  transportation  and  while 
awaiting  favorable  conditions  for  germina  ion,  it  must  be  aban- 
doned at  some  time  in  order  that  the  embryo  may  develop  into 
the  plant,  the  production  of  which  is  the  seed's  chief  function. 
By  germination  of  a  seed  is  meant  that  awakening  from  the  rest- 
ing condition  in  which  the  young  plant  shows  practically  no 
signs  of  life  to  a  state  of  active  growth.  The  term  germination 
is  used  in  different  ways,  being  used  to  designate  the  beginning 
growth  of  such  structures  as  a  pollen  tube,  fertilized  egg,  and 
spore,  but  in  each  case,  however,  it  refers  to  the  initial  growth. 
Seeds  are  considered  germinated  when  the  radicle  and  plumule 
have  broken  through  and  project  beyond  the  seed  coverings, 
although  germination  is  not  complete  until  the  little  plant  is  able 
to  live  independently  of  the  stored  food  of  the  seed. 

Conditions  Necessary  for  Germination.  —  The  awakening  of 
the  seed  into  active  growth  depends  upon  the  presence  of 
warmth,  moisture,  and  oxygen.  Germination  is  so  dependent 
upon  these  three  external  factors  that,  if  either  is  lacking  though 
the  other  two  are  properly  supplied,  there  will  be  very  little  or  no 
germination.  Among  different  seeds,  the  degree  of  temperature 
and  the  amount  of  moisture  and  oxygen  required  for  the  best 
germination  vary. 

Temperature  Requirement.  —  Seeds  vary  more  in  the  temper- 
ature required  for  germination  than  in  any  other  factor.  Through 
experience  we  have  learned  that  among  farm  and  garden  seeds 
there  are  different  temperature  requirements  for  germination, 
and  that  the  time  of  season  at  which  different  seeds  should  be 
planted  must  be  chosen  accordingly.  Thus  Oats,  Wheat,  and  Red 
Clover  seedi^,  which  have  a  low  temperature  requirement,  can  be 
planted  in  the  early  spring  or  late  fall  when  the  weather  and  soil  are 


90 


GERMINATION   OF  SEEDS:    SEEDLINGS 


cool,  but  if  Corn  or  Melons,  which  have  a  high  temperature  re- 
quirement, are  planted  before  the  weather  and  ground  are  warm 
they  will  decay  and  have  to  be  replanted.  In  considering  tempera- 
ture in  relation  to  germination,  three  temperatures  are  usually 
noted;  the  minimum,  the  lowest  temperature  at  which  germina- 
tion will  occur;  the  optimum,  the  temperature  most  favorable 
for  germination;  and  the  maximum,  or  highest  temperature  per- 
mitting germination.  As  the  following  table  shows,  these  tem- 
peratures are  very  different  for  different  seeds,  sometimes  differ- 
ing as  much  as  25°  or  30°  (Fahrenheit). 


GERMINATION   TEMPERATURES 

FAHRENHEIT) 

Kind  of  seeds. 

Minimum. 

Optimiun. 

Maximum. 

Oats                                          

Deg. 
32-41 
32-41 
41-51 
32-41 
32-41 
41-51 
51-61 
60-65 
60-65 

Deg. 

77-  SS 
77-  SS 
99-111 
77-  88 
77-88 
93-111 
93-111 
88-  99 
88-  89 

Deg. 

88-  99 

Wheat   Rye                                   .    .  . 

88-108 

111-122 

Red  Clover 

99-112 

Peas              

88-  98 

111-122 

111-122 

Musk  Alelon               

111-122 

Cucumber                             

111-122 

Germination,  which  proceeds  most  rapidly  at  the  optimum 
temperature,  decreases  in  rate  as  the  temperature  approaches 
the  minimum  or  maximum  as  the  following  table  shows  in 
case  of  Corn,  in  which  the  time  required  for  the  radicle  to  break 
through,  though  only  2  days  at  the  optimum  temperature,  was 
10  days  in  a  temperature  near  the  minimum.  In  the  majority  of 
cases,  the  temperature  of  the  soil  in  which  seeds  are  planted  is 
somewhat  below  the  optimum  and,  consequently,  if  the  soil  tem- 

EFFECT  OF  TEMPERATURE  ON  RATE  OF  GERMINATION 


Germinating  Period  in  Hours. 

Temperature,  °  F. 

Indian  Corn. 

Red  Clover. 

42 

240 

180 

55 

144 

32 

75 

56 

24 

87 

48 

24 

102.6 

48 

24 

111.2 

80 

MOISTURE   REQUIREMENT  91 

perature  is  lowered  as  it  often  is  by  heavy  rains  which  fill  the  soil 
with  water  or  by  days  of  cool  cloudy  weather,  germination  is 
either  very  slow  or  prevented  as  is  well  known  to  every  farmer 
and  gardener. 

Moisture  Requirement.  —  The  amount  of  moisture  required 
for  germination  is,  in  general,  that  which  will  completely  saturate 
and  soften  the  seeds.  The  water  absorbed  saturates  the  cell 
walls  and  starch  grains,  and  fills  the  living  cells  of  the  embryo 
and  all  empty  spaces  that  exist  in  the  seed.  Although  the  amount 
of  water  required  to  saturate  different  seeds  varies,  it  is  always  a 
large  per  cent,  sometimes  more  than  100  per  cent  of  the  dry 
weight  of  the  seed,  as  shown  in  the  table  below.  Reckoning  in 
pounds  from  the  percentages  given  in  the  talkie,  100  lbs.  of  Corn 
after  being  soaked  for  germination  may  weigh  144  lbs.  and  100 
lbs.  of  White  Clover  seeds  after  soaking  may  weigh  226.7  lbs. 

WATER  ABSORBED   BY   GERMINATING   SEEDS 


Seeds. 

Per  cent  of  water 
absorbed  in 
germination. 

44 

Wheat 

45  5 

Buckwheat 

46  9 

Rye 

57.7 

Oats 

59.8 

White  Beans 

92.1 

Peas                

106.8 

Red  Clover    

117.5 

Sugar  Beet    

120.5 

White  Clover 

126.7 

Most  seeds,  though  not  all,  swell  as  water  is  absorbed,  some- 
times more  than  doubling  their  dry  size.  In  fact,  the  per  cent 
of  increase  in  volume  is  often  greater  than  the  per  cent  of  water 
absorbed,  as  in  case  of  the  Pea  which  may  increase  in  volume 
167  per  cent  while  absorbing  only  enough  water  to  increase  its 
weight  about  100  per  cent. 

If  seeds  are  confined  in  a  space  which  they  fill  when  dry,  their 
swelhng  may  exert  a  force  of  several  hundred  pounds  and  often 
sufficient  to  break  strong  containers.  This  force  is  sometimes 
used  in  opening  skulls  in  anatomical  laboratories,  in  which  case 
the  skulls  are  filled  with  dry  Peas,  which  after  being  moistened 
swell  and  force  the  bones  apart. 


92 


GERMINATION   OF   SEEDS:    SEEDLINGS 


Oxygen  Requirement.  —  Although  seeds  are  in  the  optimum 
temperature  and  properly  supplied  with  moisture,  they  will  usu- 
ally not  germinate  unless  oxygen  is  supplied,  as  is  often  demon- 
strated in  the  laboratory  by  the  use  of  some  substance  to  absorb 
the  oxygen  in  the  germinator  or  by  replacing  the  air  in  the  ger- 
minator  with  hydrogen,  nitrogen,  or  some  other  substance,  so  that 
oxygen  is  excluded.  (Fig.  91.)  However,  since  the  air  is  about 
one-fifth  oxygen,  seeds  receive  enough  oxygen  to  germinate  well 
if  only  air  is  supplied,  although  germination  is  often  hastened 


W 


i 


-s-  - 


Fig.  91.  —  The  two  U-shaped  tubes,  which  contain  soaked  seeds  (s)  on 
moist  blotting  paper  at  their  stoppered  ends,  are  alike  except  that  in  B  the 
open  end  of  the  tube  is  in  pyrogallate  of  potash,  which  absorbs  the  oxygen 
from  the  air  in  the  tube,  while  in  A  the  open  end  of  the  tube  is  in  pure  water, 
in  which  case  the  oxygen  still  remains  in  the  air  of  the  tube.  The  seeds  ger- 
minate well  in  A  but  not  in  B. 


when  the  amount  of  oxygen  is  increased  artificially.  For  exam- 
ple, in  an  experiment  Wheat,  requiring  4  to  5  days  to  germinate 
in  the  air,  germinated  in  3  days  in  pure  oxygen.  There  are  a 
few  seeds,  however,  which  begin  to  germinate  without  oxygen,  but 
they  soon  die  unless  oxygen  is  supplied. 

For  lack  of  oxygen  seeds  germinate  poorly  when  planted  in  the 
soil  so  deeply  that  not  enough  air  is  accessible,  or  when  planted 
in  soils  with  their  pores  so  full  of  water  that  the  circulation  of  the 
air  is  prevented. 


CHANGES   IN  THE   STORED   FOOD  93 

Germinative  Processes 

Seeds  need  water,  oxygen,  and  warmth  in  germination  because 
upon  these  external  factors  the  internal  germinative  processes 
depend.  For  dissolving  and  transporting  foods  water  is  indis- 
pensable; the  occurrence  of  certain  chemical  processes  depends 
upon  oxygen;  and  in  order  for  both  chemical  and  physical  proc- 
esses to  be  suitably  active,  as  previously  shown  (page  90), 
warmth  is  required. 

Changes  in  the  Stored  Food.  —  The  first  of  the  germinative 
processes  has  to  do  with  the  digestion  and  translocation  of  the 
stored  foods.  Whether  stored  outside  of  the  embryo  or  in  the 
cotyledons,  the  stored  foods,  until  brought  nearer,  are  beyond 
the  absorptive  reach  of  the  cells  of  the  plumule  and  radicle  where 
they  are  most  needed.  But  unless  foods  are  in  solution,  which  is 
the  only  form  in  which  they  can  pass  through  the  walls  and  proto- 
plasm of  cells,  they  can  not  move  from  one  region  of  a  plant  to 
another.  Therefore,  since  starch,  fat,  and  protein,  which  are  the 
chief  storage  foods  of  seeds,  are  not  readily  soluble  in  water,  they 
must  be  changed  to  sugar,  fatty  acids,  peptones,  or  other  soluble 
forms  before  being  transported.  However,  this  digestive  process 
occurs  not  only  in  seeds  but  also  in  all  plant  regions  where  foods 
are  transported,  and  also  in  animals  it  has  its  likeness  in  the  diges- 
tive process  by  which  foods  are  made  soluble,  so  that  they  can 
pass  through  the  walls  of  the  alimentary  canal  to  the  blood,  which 
carries  them  in  solution  throughout  the  body.  Both  the  digestion 
and  transportation  of  the  stored  foods  are  quite  noticeable  during 
the  germination  of  some  large  seeds,  as  in  case  of  Corn  in  which 
the  endosperm  becomes  watery  and  disappears  as  germination 
proceeds,  or  in  case  of  Beans  where  the  cotyledons  in  which  the 
food  is  stored  gradually  shrink  as  the  young  plant  develops. 

The  digestive  process  in  plants  as  well  as  in  animals  is  per- 
formed by  special  substances  known  as  enzymes,  which  in  case  of 
the  seed  are  secretions  of  the  embryo.  Enzymes  occur  in  solu- 
tion, either  dissolved  in  water  or  in  protoplasm,  in  all  parts  of  the 
plant  where  they  either  initiate  or  hasten  chemical  changes. 
They  are  exceedingly  important  substances  because  upon  them 
the  majority  of  chemical  changes  in  plants  depend.  They  are 
specific  in  their  action,  that  is,  as  a  rule,  each  enzyme  acts  on  only 
one  kind  of  a  substance,  and  is  concerned  with  only  one  or  two 


94  GERMINATION   OF  SEEDS:    SEEDLINGS 

chemical  changes.  Consequently,  the  kinds  of  enzymes  are 
almost  as  numerous  in  the  plant  as  the  kinds  of  substances  to  be 
acted  upon.  Thus  for  changing  starch  into  sugar  there  is  the 
enzyme  known  as  diastase  which  is  especially  active  in  seeds,  but 
common  in  other  plant  organs  and  in  animal  saliva.  An  enzyme 
secreted  by  the  Yeast  Plant  and  called  zymase  acts  on  sugar, 
forming  besides  alcohol,  carbon  dioxide  which  puffs  up  the  dough 
when  Yeast  is  used  in  bread-making.  This  enzyme  also  occurs 
in  seeds,  fruits,  and  other  plant  organs.  Lipase  converts  fats  into 
soluble  fatty  acids,  and  pepsin  changes  insoluble  proteins  into 
peptones  and  other  soluble  forms.  Then  there  are  oxidases,  en- 
zymes which  oxidize  substances  as  the  name  suggests,  and  perox- 
idases which  take  oxygen  away  from  compounds,  and  many  other 
enzymes  which  play  an  important  role  in  the  chemical  activities 
of  the  plant.  The  exact  chemical  nature  of  enzymes  has  never 
been  determined  because  of  the  difficulty  in  separating  them  from 
other  protoplasmic  substances  which  enter  into  and  thus  compli- 
cate the  analysis.  Nevertheless,  there  is  much  evidence  that 
enzymes  are  protein-like  substances.  One  striking  feature  of  an 
enzyme  is  that  it  does  not  enter  into  the  chemical  action  which  it 
causes,  and,  therefore,  a  small  quantity  of  an  enzyme  can  keep 
a  chemical  action  going  until  a  large  quantity  of  a  substance  is 
changed. 

Although  all  hving  cells,  whether  in  the  embryo  or  elsewhere, 
produce  enzymes,  sometimes,  however,  certain  cells  have  the 
secretion  of  enzymes  as  their  special  function,  as  in  Corn,  Wheat, 
and  other  seeds  of  the  Grass  type,  where  the  epithehal  layer  of  the 
scutellum  has  for  its  special  function  the  secretion  of  the  diastase 
and  other  enzymes  which  are  necessary  for  converting  the  endo- 
sperm into  soluble  forms. 

Transportation  of  Soluble  Foods.  —  After  the  foods  are  made 
into  solul)le  forms  and  dissolved  in  the  water  present,  they  pass 
from  one  region  of  the  plant  to  another  by  the  physical  processes 
known  as  diffusion  and  osmosis.  Diffusion  is  probably  better 
known  among  gases  where  the  spread  of  odors  through  a  house, 
the  fragrance  of  flowers  through  gardens,  and  smoke  through  the 
air  are  everyday  illustrations  of  it.  The  spread  of  indigo,  ink, 
or  any  substance  like  salt  and  sugar  through  the  water  in 
which  they  are  dissolving  illustrates  it.  By  diffusion  substances, 
whether  dissolved  in  a  gas  or  a  liquid,  spread  farther  and  farther 


ELABORATION   OF   FOODS   INTO   PLANT  STRUCTURES      95 

from  the  place  where  they  entered  the  dissolving  medium,  and 
thus  toward  those  regions  where  they  are  less  concentrated.  In 
case  a  number  of  substances  are  in  solution  at  the  same  time,  each 
diffuses  independently  of  the  others.  When,  for  example,  sugar, 
salt,  and  ink  are  dissolved  in  a  vessel  of  water  at  the  same  time, 
each  diffuses  to  all  parts  of  the  vessel  independently  of  the  others 
and,  consequently,  the  substances  become  thoroughly  mixed  just 
as  the  oxygen,  nitrogen,  carbon  dioxide,  and  other  gases  of  the 
air  by  diffusion  tend  to  thoroughly  mix.  It  is  apparent  then  in 
case  of  the  seed  that  foods  in  a  concentrated  solution  in  the  endo- 
sperm or  cotyledons  will  diffuse  to  the  radicle  and  plumule,  where 
the  food,  by  being  constantly  removed  from  the  solution  to  be 
built  into  plant  structures,  is  kept  less  concentrated. 

Osmosis  mentioned  as  another  process  involved  in  the  trans- 
portation of  foods  is  also  a  diffusion,  but  differs  from  the  ordinary 
diffusion  just  described  in  that  it  takes  place  through  a  membrane 
which  alters  the  rate  of  the  diffusion  of  different  substances  by 
allowing  some  to  pass  through  it  more  readily  than  others.  It  is 
by  this  kind  of  diffusion  that  substances  pass  into  and  out  of  liv- 
ing cells,  in  which  case  the  membrane  through  which  the  sub- 
stances must  diffuse  is  the  modified  border  of  the  protoplasm. 
Thus,  although  foods  depend  much  upon  ordinary  diffusion  for 
transportation  when  not  passing  through  meml^ranes,  in  entering 
or  leaving  living  cells  they  must  also  depend  upon  osmosis,  the 
nature  and  principles  of  which  are  more  thoroughly  discussed  in 
connection  with  the  cell  (Chapter  VII). 

The  Elaboration  of  Foods  into  Plant  Structures.  —  In  the  early 
stages  of  germination  the  radicle  and  plumule  elongate  by  the  elon- 
gation of  the  cells  already  present,  but  soon,  however,  in  certain 
regions,  mainly  at  or  near  the  tip  of  the  radicle  and  plumule,  there 
begins  cell  division  followed  by  elongation,  growth,  and  forma- 
tion of  tissues  —  the  processes  upon  which  the  continued  develop- 
ment of  the  young  plant  depends.  Throughout  these  processes 
foods  are  elaborated:  (1)  into  materials  to  thicken  the  cell  walls 
as  they  become  thinner  in  stretching;  (2)  into  protoplasm  which 
must  increase  as  cells  grow  and  divide;  (3)  into  woody  and  other 
elements  for  strength  and  conduction;  (4)  into  fatty  and  waxy 
substances  and  cell  thickenings  for  protection;  and  (5)  into  the 
various  materials  which  are  peculiar  to  food-making,  reproduc- 
tive, absorbing,  secreting,  and  other  structures  which  plants  form 


96  GERIMIXATION  OF  SEEDS:    SEEDLINGS 

during  their  dcvolopment.  But  the  transformation  of  foods  into 
the  various  structural  elements  of  the  plant  involves  chemical 
reactions  which  take  place  only  when  there  is  energy  supplied. 
This  brings  us  to  another  process  called  respiration  by  which  the 
energy  required  for  the  chemical  changes  involved  in  changing 
foods  into  cell  walls,  protoplasm,  and  other  structures  is  secured. 

Respiration  in  plants,  just  as  in  animals,  is  an  oxidation  process 
in  which  some  food  or  other  elements  are  burned,  as  we  commonly 
say,  with  the  result  that  oxygen  is  required  and  energy,  carbon 
dioxide,  and  water  vapor  are  produced.  Respiration  occurs  only 
within  the  cell  in  connection  with  which  it  will  be  more  fully  dis- 
cussed. But  since  there  is  no  place  where  respiration  is  more  in 
evidence  than  in  germination  where  the  cells  are  extremely  ac- 
tive, some  of  its  features  should  be  noted  in  connection  with  that 
process.  Furthermore,  much  about  germination  can  not  be  un- 
derstood until  something  is  known  about  respiration. 

Cells,  like  an  electric  motor,  steam  engine,  etc.,  can  not  do  work 
unless  they  have  energy.  Some  cells,  like  the  green  cells  of  leaves, 
are  able  to  utihze  the  sun's  energy  for  some  kinds  of  work;  but 
when  cells  are  not  specially  provided  with  pigments  for  utilizing 
the  sunlight,  they  have  to  depend  entirely  upon  the  energy 
which  they  produce  within  themselves.  In  the  sugar  and  other 
foods  of  the  seed  there  is  much  latent  energy  which  can  be 
released  as  active  energy  by  oxidizing  these  substances,  which 
are  thereby  broken  into  simpler  compounds  of  which  carbon  diox- 
ide and  water  are  the  simplest  and  most  noticeable  ones.  It  is 
this  oxidizing  of  substances,  so  that  their  stored  energy  is  re- 
leased, that  constitutes  respiration,  which  necessarily  must  be 
accompanied  by  a  consumption  of  oxygen  and  the  production 
of  simpler  compounds.  It  is  now  clear  why  seeds  do  not  ger- 
minate well  when  oxygen  is  excluded  as  the  experiment  in  Figure 
91  demonstrates.  Although  most  of  the  energy  released  is  used 
in  carrying  on  the  work  of  the  cell,  some,  however,  escapes  as 
heat,  which,  like  the  liberation  of  carbon  dioxide  and  water  vapor, 
indicates  that  respiration  is  going  on. 

Respiration  in  seeds  is  easily  demonstrated  by  germinating 
seeds  in  a  closed  jar,  in  which  the  production  of  heat  and  carbon 
dioxide  with  the  accompanying  loss  of  oxygen,  and  the  accumu- 
lation of  moisture  can  be  demonstrated.  By  germinating  seeds, 
such  as  Peas  or  Beans,  in  a  closed  vessel  in  which  a  thermometer 


ELABORATION  OF  FOODS  INTO  PLANT  STRUCTURES   97 


is  inserted,  the  temperature  of  the  enclosed  air  may  be  raised  10°  C. 

and  sometimes  20°  C.  by  the  heat  of  res- 
piration; and  the  oxygen  of  the  enclosed 

air  will  usually  be  so  nearly  used  up  that 

the  flame  of  a  burning  match  or  splinter 

is  extinguished  when  inserted  into  the 

jar.  {Fig.  92.)  To  demonstrate  the  ac- 
cumulation of  carbon  dioxide,  one  may 

pour  lime  water  into  the  jar  where  the 

seeds  are  germinating,  in  which  case  the 

calcium    hydroxide    of    the   lime    water 

unites  with  the  carbon  dioxide  of   the 

enclosed  air,  forming  calcium  carbonate 

which  is  insoluble  and  when  abundant 

gives  the  solution  a  milky  appearance. 

Since  the  amount  of  carbon  dioxide  in 

ordinary  air  is  not  sufficient  to  give  a 

perceptible  precipitate,  the  milky  ap- 
pearance, therefore,  indicates  that  much 

carbon  dioxide  has  been  added  to  the 

enclosed  air.     Again,  the  carbon  dioxide 

liberated   in  germination   can   be  quite 

accurately  measured  by  drawing  the  air 
from  over  germinat  ng  seeds  through  a 
solution  of  potassium  hydroxide,  where 
the  carbon  dioxide  is  caught  and  its 
weight  calculated  from  the  increased 
weight  of  the  solution.  However,  this 
involves  careful  weighing  as  well  as  see- 
ing to  it  that  the  carbon  dioxide  already 
present  in  the  air  is  removed  before  the 
air  enters  the  germinator,  and  that  the 
increased  weight  of  the  potassium  hy- 
droxide is  not  partly  due  to  added  mois- 
ture. This  method  discloses  that  many 
cubic  centimeters  of  carbon  dioxide  may 
be  liberated  by  a  small  quantity  of  ger- 
minating seeds,  as  shown  by  the  experi- 
ment in  which  3  Beans  with  a  dry  weight 


periment  to  demonstrate 
that  heat  is  produced  by 
germmatmg  seeds.  The 
bottle  A  contains  germi- 
nating seeds,  while  the 
bottle  B  contains  only 
moist  cotton.  The  higher 
temperature,  commonly 
shown  by  the  thermometer 
in  bottle  .1,  demonstrates 
that  germination  is  ac- 
companied by  the  pro- 
duction of  heat.  If  the 
bottles  are  protected 
against  the  loss  of  heat,  or 
if  bottles  like  "Thermos" 
bottles,  which  have  double 
wall^  with  air-space  be- 
tween, are  used,  the  re- 
sults are  much  better. 


of  only  1  gram  produced  9|  cubic  centimeters  of  carbon  dioxide 


98  GERMINATION   OF  SEEDS:    SEEDLINGS 

during  a  germinative  period  of  only  48  hours.  That  moisture 
is  hberated  during  germination  is  obvious,  for  the  air  in  a  closed 
gcrminator  often  l^ecomes  so  saturated  that  moisture  precipitates 
on  the  walls  of  the  germinator. 

When  green  seeds,  green  hay,  or  any  plant  portions  in  which 
the  cells  are  quite  active  are  massed  together,  so  that  the  heat  and 
moisture  are  retained,  they  often  become  very  warm  and  moist 
due  partly  to  their  own  respiration  and  partly  to  that  of  the  micro- 
organisms present.  The  so-called  "sweating"  of  grains  in  the 
stack  or  bin  and  the  heating  in  the  bin  when  the  grain  becomes 
damp  due  to  leaks  are  phenomena  connected  with  respiration. 

Summary.  —  In  germination  of  seeds  the  following  things  take 
place:  (1)  the  absorption  of  water  which  softens  the  seed  cover- 
ings and  acts  as  a  dissolving  and  transporting  medium  of  foods; 
(2)  the  secretion  of  enzymes  which  digest  the  foods  and  assist  in 
other  processes;  (3)  the  transference  of  foods  by  diffusion  and 
osmosis;  (4)  respiration  which  supplies  energy  for  the  elabora- 
tion of  foods  into  plant  structures  and  is  accompanied  by  the  ab- 
sorption of  oxygen  and  the  production  of  carbon  dioxide,  water 
vapor,  and  some  heat;  and  (5)  the  growth  of  the  radicle  and 
plumule,  resulting  in  the  breaking  of  the  seed  coverings  and  the 
establishment  of  the  young  plant  in  the  soil  and  sunlight. 

Testing  the  Germinative  Capacity  of  Seeds 

The  loss  in  crop  and  labor  when  poor  seed  is  used  may  be  so 
serious  that  no  one  can  afford  to  plant  seeds  with  a  doubtful  ger- 
minative capacity.  It  is  not  enough  for  seeds  to  germinate,  but 
they  should  have  vigorous  embryos,  so  that  they  will  germinate 
quickly  and  thus  rapidly  pass  through  the  deHcate  stage  in  which 
the  young  plant  is  likely  to  be  destroyed  by  insects.  Fungi,  bad 
weather,  and  unfavorable  soil  conditions. 

In  testing  the  germinative  capacity,  as  in  determining  the  im- 
purities of  a  quantity  of  seeds,  decision  is  based  upon  the  results 
obtained  with  a  comparatively  small  number  of  the  seeds  as  a 
sample.  In  case  of  small  seeds,  such  as  Oats,  Wheat,  Barley,  and 
Clover,  Alfalfa,  and  Grass  seeds,  tests  are  ordinarily  made  with 
two  lots  consisting  of  200  seeds  each  and  free  from  impurities. 
In  Corn  it  is  customary  to  use  6  kernels,  2  from  near  the  tip, 
2  from  the  butt,  and  2  from  the  middle  of  the  ear,  with  the 
kernels  of  each  pair  selected  from  rows  as  far  apart  as  possible. 


TESTING    THE   GERMINATIVE   CAPACITY   OF   SEEDS       99 

There  are  a  number  of  genninators  on  the  market,  but,  if  one 
is  not  available,  a  box  of  moist  soil  or  sand,  or  moist  rags  which 
are  rolled  up  with  the  seeds  within  are  good  genninators  when 
properly  handled.  {Fig.  93.)  A  very  good  germinator  is  made 
with  two  dinner  plates  and  blotting  paper  as  shown  in  Figure  9^. 

During  the  test  a  temperature  suitable  for  the  germination  of 
the  kind  of  seeds  involved  must  be  maintained.  Some  prefer  to 
keep  the  temperature  near  that  of  the  soil,  so  as  to  more  nearly 


Fig.  93.  —  Doll  rag  testers,  consisting  of  moist  rags  properly  labeled  and 
rolled  up  with  the  seeds  withm.     After  H.  D.  Hughes. 

imitate  the  soil  conditions  under  which  most  seeds  do  not  germi- 
nate so  well  as  they  do  in  germinators.  The  germinator  should 
be  opened  each  day  to  note  the  germinated  seeds  and  to  allow  the 
entrance  of  fresh  air,  if  ventilation  is  not  otherwise  provided.  At 
the  end  of  the  germinative  period,  the  results  are  usually  ex- 
pressed in  percentages  found  by  dividing  the  number  of  germinated 
seeds  by  the  number  in  the  lot  and  multiplying  by  100.     Thus  if 

190  of  a  lot  of  200  germinated,  -^^^^^^^  =  95  per  cent.     The 

percentage  of  germination  will  vary  for  different  lots  and  the 


100 


(JEimiXATIOX   OF   SE?]I)S:    SEEDLINGS 


greater  (he  number  of  lots  tested,  the  more  the  results  will  be 
checked  and,  accordinglj-,  the  safer  will  be  the  conclusions. 

In  estimating  the  germinative  capacity  of  seeds,  the  time 
allowed  for  germination  must  be  considered;  for  seeds  having 
weak  embryos  and,  therefore,  unfit  for  planting  may  give  a  high 
percentage  of  germination  if  allowed  enough  time.  It  is,  there- 
fore, necessary  to  fix  a  time 
limit,  and  in  doing  so  the  ger- 
minative speed  characteristic 
of  the  type  of  seeds  involved 
and  the  temperature  of  the 
germinator  must  be  consid- 
ered ;  for  some  seeds  naturally 
germinate  more  slowly  than 
others,  and  the  effects  of  low 
and  high  temperatures  on  ger- 
mination are  already  known 
to  the  student  (page  90). 
Furthermore,  kinds  of  seeds 
differ  so  much  in  germinative 
capacity  that  a  percentage  of 
germination  considered  good  for  one  kind  of  seeds  would  be  con- 
sidered poor  for  another.  Thus  70  per  cent  germination  is  good 
for  Parsnip  seeds  but  very  poor  for  Wheat  or  Corn.  In  the 
following  table  ^  the  number  of  days  in  which  the  seeds  should 
germinate  enough  to  show  their  germinative  capacity,  and  the 
percentages  of  germination  considered  good  for  first-class  fresh 
seeds,  one  year  with  another,  are  given. 


Fig.  94.  —  Simple  germinator.     A, 
closed.    B,  open.    After  F.  H.  Hillman. 


Red  Clover 

Alsike  Clover 

White  Clover 

Alfalfa 

Timothy 

Bluegrass  (Kentucky), 

Millet 

Wheat 

Oats 

Barley 

Flax 

Corn 


Germinatioi 
period,  days 


Good 

germination, 

per  cent. 


1  Testing  Farm  Seeds  in  the  Home  and  in  the  Rural  Schools. 
Bulletin  ^28,  U.  S.  Dept.  of  Agriculture. 


Farmers' 


(,.,«>- ^^"^^ 


SEEDLINGS 
Seedlings 


lUl 


After  the  radicle  and  plumule  have  escaped  from  the  seed  cov- 
erings, the  young  plant  passes  into  the  seedling  stage,  which  lasts 
until  the  young  plant  becomes  entirely  self-supporting,  that  is, 


Fig.  95.  —  Early  stages  in  the  development  of  the  Corn  seedling.  A, 
section  through  kernel,  showing  cotyledon  (c),  radicle  (r),  and  i)lumule  (p). 
B,  after  germination  with  radicle  or  primary  root  (r)  and  plumule  {p)  much 
elongated.  C,  radicle  (r)  and  plumule  (p)  much  further  developed;  s,  sec- 
ondary roots;  /,  leaves;  /,  coleoptilo. 

until  it  no  longer  receives  any  of  its  food  supi)ly  from  the  seed. 
From  the  seedling  stage  the  plant  passes  into  the  adult  stage,  ex- 
cept in  trees  where  a  sapling  stage  occurs.     However,  the  division 


102 


GERMIXATIOX  OF  SEEDS:    SEEDLINGS 


of  a  plant's  life-C3'clc  into  successive  stages  is  somewhat  artificial, 
for  the  stages  so  overlap  that  they  can  not  be  separated.  In  tJiis 
presentation  we  are  chiefly  concerned  with  the  seedling  stage  — • 

the  stage  in  which  plants 
present  differences  that 
sometimes  must  be  reck- 
oned with  in  choosing 
proper  methods  of  plant- 
ing and  cultivating,  and 
that  often  explain  the 
peculiar  features  of  the 
plant  in  the  adult  stage. 
Among  our  cultivated 
plants  there  are  four  rather 
distinct  types  of  seedlings 
as  those  of  the  Grasses, 
Onion,  Beans,  and  Peas 
illustrate. 

Seedlings  of  the  Grass 
Type.  —  The  seedhngs  of 
all  Grasses  are  so  similar 
in  type  that  their  essential 
features  may  be  learned 
by  studying  the  seedling 
stage  of  Corn.  From  Fig- 
ure .  95,  showing  the  de- 
velopment of  the  Corn 
seedling,  it  is  seen  that  the 
radicle  develops  directly 
downward,  forming  the 
first  root  called  'primary 
root  from  which  secondary 
roots  arise  as  branches. 
However,  not  all  second- 
ary roots  arise  at  this  time 
from  the  radicle,  for  some 
often  grow  out  from  the  stem  just  above  or  below  the  cotyledon. 
The  plumule,  although  developing  more  slowly  at  first  than  the 
radicle,  soon  breaks  through  its  sheath-like  covering  (coleoptile) 
and  rapidly  elevates  its  leaves  to  the  light.     As  the  plumule  is 


Fig.  96.  —  A  later  stage  of  the  Corn  seed- 
ling, g,  ground  line;  p,  plumule;  a,  first 
node  with  permanent  root  system;  b,  portion 
of  stem  between  the  first  node  and  kernel; 
k,  kernel;  r,  radicle  or  primary  root;  s,  sec- 
ondary roots  of  the  primary  root  system; 
d,  permanent  root  system;  c,  coleoptile. 
About  half  natural  size. 


SEEDLINGS  OF   THE   GRASS  TYPE  103 

unfolding  its  first  leaves  to  the  light,  a  zone,  called  a  node,  is 
formed  at  its  base  about  2  inches  under  the  surface  of  the  soil, 
and  from  this  node  and  others  soon  forming  above  it,  there  arise 
roots  of  a  much  larger  and  stronger  type  than  those  formed  from 
the  radicle  and  from  the  stem  in  the  region  of  the  cotyledon. 
These  secondary  roots,  which  are  outgrowths  of  the  plumule  since 
they  arise  from  its  nodes,  constitute  the  permanent  root  system, 
which  as  the  name  suggests  remains  active  as  an  anchoring  and 


Fig.  97.  —  Diagram  showing  the  effect  of  planting  Corn  at  different  depths. 
g,  ground  line;  p,  permanent  root  system,  which  always  develops  at  about  the 
same  distance  under  the  surface;  a,  temporary  region  of  the  stem,  which  is 
much  longer  in  deep  planting;  /c,  kernel;  /,  temporary  root  system.  Modified 
from  "Elementary  Principles  of  Agriculture"  by  Ferguson  and  Lewis. 

absorptive  system  as  long  as  the  plant  lives.  After  the  permanent 
roots  are  established  (about  10  days  after  planting)  the  fii'st 
roots,  which  are  known  as  the  temporary  roots  since  they  serve 
the  plant  only  till  the  permanent  roots  are  established,  develop 
no  further  and  remain  as  vestigial  structures  until  they  finally 
disappear. 

Also  included  among  the  temporary  structures  is  the  portion 
of  stem  between  the  first  node  and  kernel.  (Fig.  96.)  During 
the  early  stage  of  germination,  this  stem  portion  performs  two 


104 


GERMINATION   OF   SEEDS:    SEEDLINGS 


important  functions:  (1)  by  its  elongation  the  plumule  is  assisted 
in  reaching  above  the  soil;  and  (2)  through  it  the  endosperm 
and  substances  absorbed  by  the  temporary  roots  reach  the  plu- 


FlG 

ILshed 

tion;   k,  grain;   (,  temporary  root  system 


98.  —  Seedling  of  Wheat  after  the  permanent  root  system  is  estab- 
g,  ground  hne;    /),  permanent  root  system;    a,  temporary  stem  por- 


About  half  natural  size. 


mule.  But  after  the  permanent  roots  are  well  established,  there 
is  no  longer  any  need  for  this  stem  region,  which  now  being  with- 
out a  function   makes  no   further  development.      Nevertheless 


SEEDLINGS  OF  THE  GRASS  TYPE 


105 


in  connection  with  it,  there  is  a  principle  which  is  reckoned 
with  in  growing  certain  plants  of  the  Grass  type.  According 
to  the  depth  of  planting  this  temporary  stem  region  is  long  or 
short.  (Fig.  97.)  This  is  due  to  the  fact  that  the  first  node  and, 
consequently,  the  first  of  the  permanent  roots  are  always  estab- 
lished about  the  same  distance  under  the  surface  of  the  soil,  re- 
gardless of  the  depth  at  which  the  seed  was  planted.     Therefore, 


Fig.  99.  —  Stages  in  the  development  of  the  Onion  seecUing.  A,  section 
through  an  Onion  seed  showing  endosperm  {en)  and  embryo  {c)  with  the 
hypocotyl  {h)  and  cotyledon  (c)  indicated.  B,  seed  germinating;  g,  ground 
Une;  s,  seed;  c,  cotyledon;  h,  hypocotyl;  r,  radicle.  C,  seedling  more  de- 
veloped; c,  cotyledon  which  is  being  pulled  out  of  the  seed;  h,  hypocotyl; 
r,  radicle;  /,  first  leaf.  D,  a  later  stage  of  the  seedling  with  cotyledon  free 
from  the  seed  and  permanent  root  system  (p)  developing. 

a  deep  permanent  root  system,  which  is  often  desirable  in  order 
that  the  plant  may  withstand  drought,  is  not  secured  by  deep 
planting  —  a  fact  which  has  been  well  demonstrated  in  case  of 
Corn  and  the  small  grains.  Moreover,  if  the  seed  is  planted  too 
deeply,  its  food  and  energy  may  be  exhausted  ])efore  the  plumule 
reaches  the  light,  in  which  case  the  seedling  is  unal>le  to  continue 
its  development. 

However,  after  the  permanent  roots  are  established  they  may 


106 


GERMINATION  OF  SEEDS:    SEEDLINGS 


be  put  deeper  in  the  soil  by  adding  dirt  around  the  plant.  In 
semi-arid  regions  where  a  deep  permanent  root  sj'stem  is  desired, 
the  ground  is  often  listed,  that  is,  plowed  into  deep  furrows,  and 
the  Corn  planted  in  the  bottom  of  the  furrows.     Then  as  the 

furrows    are   gradually 
\^     ^;^^\  filled  in  cultivation,  the 

"^  \      {  permanent     roots     are 

buried  deeper  in  the 
soil,  where  there  is  a 
chance  for  moisture 
during  drought.  In  this 
same  connection,  one 
can  see  some  advan- 
tage in  drilling  small 
grains  in  that  the  roots 
of  the  plants  will  be 
buried  deeper  as  the 
dirt  from  the  ridges  is 
carried  into  the  drill 
furrows  during  rains 
and  thaws. 

In  the  small  grains, 
such  as  Wheat,  Oats, 
Barley,  etc.,  although 
the  temporary  system 
is  just  as  prominent  as 
in  Corn,  there  is,  how- 
ever, a  difference  of 
minor  importance  to  be 
noted  in  the  number 
of  primary  roots,  which 
is  one  in  Corn  but  two 
or  more  in  the  small 
grains.  {Fig.  98.) 
The  presence  of  the 


Fig.  100.  —  Stages  in  the  development  of  a 
Common  Bean  seedling.  A,  the  cotyledons 
(c)  being  pulled  out  of  the  ground  by  the  hy- 
pocotyl  (h).  t,  testa;  r,  radicle;  a,  root  hairs; 
g,  ground  hne.  B,  the  hypocotyl  has  straight- 
ened, and  the  cotyledons  have  shed  the  testa 
and  spread  apart,  thus  giving  freedom  to  the 
plumule  (p).  C,  stage  with  plumule  develop- 
ing stem  and  leaves  (J),  root  system  much  en- 
larged by  secondary  roots  (s),  and  cotyledons 
(c)  shrinking  through  loss  of  stored  food. 


temporary  system,  although  occurring  in  other  plants,  is  a  not- 
able feature  of  the  Grass  seedlings.  Another  feature  to  be  noted 
is  that  the  cotyledon  remains  where  the  seed  was  placed  in 
planting,  that  is,  it  is  not  pushed  up  out  of  the  soil  by  an  elong- 
ating hypocotyl. 


SEEDLINGS  OF  THE   COMMON   BEAN   TYPE 


107 


Onion  Seedling.  —  The  seedling  of  the  Onion  represents 
another  type  of  monocotyledonous  seedUngs.  In  this  type  the 
hypocotyl  elongates  and  pushes  the  cotyledon  above  ground. 
(Fig.  99.)  As  in  the  Grass  seedlings,  the  primary  root  system 
is  temporary  —  a  feature  quite  common  in  Monocotyledons,  al- 
though in  some  it  lives  much  longer  than  in  others. 

Seedlings  of  the  Common  Bean  Type.  —  The  seedhng  of  the 
Common  Bean  is  representative  of  those  dicotyledonous  seedlings 
in  which  the  cotyledons  through  the  elongation  of  the  hypocotyl 
are  carried  above  ground,  sometimes  several  inches  or  even  a 
foot  in  some  Beans.  Squashes, 
Cucumbers,  Pumpkins,  Melons, 
Radishes,  Turnips,  Castor  Bean, 
Maples,  Ashes,  Clover,  Alfalfa, 
etc.,  besides  many  of  the  Beans 
have  this  type  of  seedling.  In 
seedlings  of  this  type  the  first  root 
system  is  usually  the  permanent 
one  and  soon  firmly  anchors  the 
hypocotyl  which  then  by  an  arch- 
ing movement  pulls  the  cotyle- 
dons out  of  the  ground  in  such  a 
way  that  they  offer  the  least  re- 
sistance in  passing  through  the 
soil  and  afford  the  most  protec- 
tion for  the  delicate  plumule. 
{Fig.  100.)  In  some  cases,  as  in 
the  Melons  and  Pumpkins,  the 
hypocotyl  also  assists  in  casting 
off  the  seed  coat,  in  which  case  the  arch  of  the  hypocotyl  pulls 
the  cotyledons  out  of  the  seed  coat  while  the  latter  structure  is 
held  in  place  by  a  peg-like  structure  of  the  hypocotyl.  {Fig.  101.) 
In  most  cases,  however,  the  seed  coat  is  torn  and  gradually 
pushed  off  by  the  growth  of  the  seedling.  Since  the  first  root 
system  is  usually  the  permanent  one,  its  depth  is  closely  related 
to  the  depth  of  planting. 

The  plumule  remains  small  and  enclosed  between  the  coty- 
ledons until  pulled  out  of  the  soil.  Then  by  a  straightening  of 
the  hypocotyl  arch  and  the  spreading  of  the  cotyledons,  it  is  fully 
exposed  to  the  light,  where  it  develops  all  of  the  plant  above  the 


Fig.  101.  —  Squash  seed  germi- 
nating, showing  the  peg  by  which 
the  seed  coat  is  held  while  the 
cotyledons  are  pulled  out  of  the 
seed  coat  by  the  arch  of  the  hy- 
pocotyl.    Somewhat  reduced. 


108 


GERMINATION  OF  SEEDS:    SEEDLINGS 


cotyledons.     Thus  most  of  the  stem  and  all  of  the  leaves,  flowers, 
and  fruit  of  the  adult  stage  are  produced  by  the  plumule. 

The  cotyledons,  which  are  commonly  fleshy  in  these  seedlings, 
enlarge  after  reaching  the  light  and  their  color  changes  to 
green,  which  with  the  presence  of  stomata  indicates  that  they 
function  to  some  extent  like  ordinary  leaves  in  the  manufacture 

of  foods.  However,  it 
is  only  a  short  time  till 
most  of  them,  espe- 
cially the  fleshy  ones, 
begin  to  show  shrink- 
age which  continues  as 
the  food  is  used  for 
growth,  until  much 
shriveled  and  dried 
the}''  fall  from  the 
plant.  In  some  cases, 
as  in  the  Buckwheat 
and  Castor  Bean  where 
the  seeds  are  albumin- 
ous, the  thin  cotyledons 
are  more  leaf-like  and 
function  like  ordinarj'^ 
leaves  for  a  consider- 
able time,  although  in 
arrangement,  shape,  or 
size  they  are  never  just 
like  ordinary  leaves  and 
never  so  long-lived. 
{Fig.  102.)  Where  the 
cotyledons  are  large, 
much  force  is  required 
to  pull  them  through 
the  soil,  and,  consequently,  when  the  ground  is  hard  or  covered 
with  a  crust,  seedlings  of  this  type  often  fail  to  develop. 

As  to  how  the  development  of  both  radicle  and  plumule  pro- 
ceeds until  the  adult  stage  is  reached,  that  depends  much  upon 
the  kind  of  plant.  In  most  cases  the  radicle  forms  a  central  root 
which,  although  prominent  at  first,  may  be  much  obscured  in  the 
adult  stage  by  large  secondary  roots  developing  from  the  base  of 


Fig.  102.  —  Seedling  of  Castor  Bean,  in  which 
the  cotyledons  persist  and  function  like  leaves 
for  some  time. 


SEEDLINGS  OF   THE   PEA   TYPE 


109 


the  stem.  In  some  plants,  as  in  Red  Clover  and  Alfalfa,  the 
radicle  forms  a  prominent  tap-root  which  enables  the  plant  to 
penetrate  deeply  into  the  soil  in  its  adult  stage. 

In  the  Morning  Glory,  where  the  stem  called  the  vine  may  be 
many  feet  in  length,  there  is  extreme  elongation  of  the  plumule. 
On  the  other  hand,  as  in  some  Clovers  and  Alfalfa,  the  plumule  and 
hypocotyl  form  a  short  thick  stem,  called  the  crown,  which  is  barely 


Fig.  103.  —  Development  of  a  Red  Clover  seedling.  A,  cotyledons  being 
pulled  out  of  the  ground  by  the  hypocotyl  {h);  r,  radicle;  a,  root  hairs;  t, 
testa;  c,  cotyledons;  g,  ground  line.  B,  a  more  advanced  stage,  showing  some 
development  of  the  plumule  (p);  b,  first  real  leaf;  d,  second  real  leaf.  C,  a 
later  stage,  showing  that  the  plumule  has  formed  more  leaves  (e)  but  has 
elongated  very  little. 

above  the  surface  of  the  ground,  and  from  which  the   branches 
arise  that  bear  the  leaves,  flowers,  and  fruit.     {Fig.  103.) 

Seedlings  of  the  Pea  Type.  —  The  seedlings  of  the  Pea  and 
Scarlet  Runner  Bean  represent  those  dicotyledonous  seedlings  in 
which  the  hypocotyl  remains  short.  Thus  the  cotyledons  remain 
underground  and  the  plumule  is  pushed  to  the  surface  by  the 
elongation  of  the  stem  of  the  epicotyl  just  as  occurs  in  the  Grass 
seedlings.     But  in  these  seedlings,  in  contrast  to  those  of  the 


no 


GERMINATION   OF  SEEDS:    SEEDLINGS 


Grass  type,  both  the  stem  of  the  epicotyl  and  the  primary  root 
system  are  usually  permanent.  In  many  seedlings  of  this  type, 
the  cotyledons  are  probably  so  much  distorted  in  connection  with 
food  storage,  that  they  could  not  function  as  leaves  if  raised  to  the 
light.  Again,  it  is  claimed  that  these  seedlings  can  come  up 
through  harder  ground  by  not  having  to  raise  their  cotyledons. 
{Fig.  104.) 


Fig.  104.  —  Seedlings  of  the  Pea,  showing  how  the  seedUng  develops  and 
the  effect  of  different  depths  of  planting,  p,  plumule;  a,  stem  portion  of 
epicotyl;  g,  ground  line;  r,  radicle.  The  seedling  at  the  right  is  so  deep  in 
the  soil  that  it  is  unable  to  push  the  plumule  out  of  the  ground. 


Size  of  Seedlings.  —  There  is  no  feature  in  which  seedlings 
vary  more  than  in  size.  This  might  be  illustrated  by  placing  the 
seedhng  of  Timothy  or  Clover  by  the  side  of  a  Coconut  seedling. 
In  general,  the  size  of  the  seedling  corresponds  to  the  size  of  the 
seed.  The  size  of  seedlings  is  reckoned  with  in  our  methods  of 
planting  different  seeds.  Thus  seeds,  like  Corn  and  Beans,  are 
planted  several  inches  deep  in  the  soil,  while  seeds,  like  those  of 
Lettuce,  Clover,  and  Timothy,  are  sown  on  the  surface,  and  cov- 
ered only  lightly  if  at  all.  In  small  seedlings  there  is  not  enough 
food  to  enable  the  plant  to  reach  through  thick  layers  of  soil. 
Tests  have  shown  that  not  many  Clover  seedlings  get  through 
the  soil  when  the  seeds  are  planted  even  2  inches  in  depth. 


SUMMARY  OF  SEEDLINGS  111 

Summary  of  Seedlings.  —  Seedlings  of  Flowering  Plants  are 
either  nionocotyledonous  or  dicotyledonous  on  the  basis  of  the 
number  of  cotyledons.  Among  the  Monocotyledons  the  tem- 
porary root  system  is  a  prominent  feature,  and  the  cotjdedon  may 
remain  in  the  ground  as  in  the  Grasses  or  be  raised  to  the  light  as 
in  the  Onion.  In  Dicotyledons  the  first  root  system  is  usually 
the  permanent  one  and  may  consist  mainly  of  a  tap-root  or  of 
many  roots  nearly  equal  in  size.  In  many  Dicotyledons  the  coty- 
ledons are  raised  to  the  light  where  they  function  to  some  extent 
like  ordinary  leaves.  The  fleshy  ones,  however,  lose  their  stored 
food  in  a  short  time  and  fall  from  the  plant.  In  some  cases,  as 
the  seedhngs  of  Buckwheat,  Morning  Glory,  and  Cotton  illustrate, 
the  cotyledons  become  more  leaf -like  and  persist  longer,  although 
they  are  always  easily  distinguished  from  true  leaves.  In  some 
Dicotyledons  the  cotyledons  remain  in  the  soil  and  the  plumule  is 
raised  to  the  light  by  the  elongation  of  the  stem  of  the  epicotyl. 


CHAPTER   VII 
CELLS   AND    TISSUES 


Structure  and  Function  of  Cells 

Position  of  the  Cell  in  Plant  Life.  —  Before  proceeding  to  the 
study  of  the  aduU  stage  of  the  plant  more  must  be  known  about 
the  cell.  If  with  a  sharp  razor  a  very  thin  section  from  any  part 
of  a  plant  is  made  and  observed  with  a  microscope,  it  will  appear 
to  be  divided  into  many  small  divisions.  A  section  through  the 
growing  portion  of  a  root  looks  like  Figure  105.     These  little 

r     divisions    with    what    they 
'^      contain  are  the  cells.     Cells 
vary  much  in  shape  and  are 
so  small  that  usually  four -or 
five  hundred  of  them  could 
be  laid  side  by  side  on  a  line 
not  more  than   an  inch  in 
length.  They  are  rarely  more 
than    T^xT   of   an   inch,    and 
sometimes  less  than  xAo  of 
an    inch    in   diameter.      Al- 
though cells  are  so  exceed- 
FiG.  105.  — A  small  portion  of  a  length-  ingly   small  nevertheless,  it 
wise  section  through  the  growing  region  is  within  them  that   all  life 
of  a  root  showing  the  cells.     Very  much   processes     take    place.      For 
^"^^'^^^^-  this   reason   cells   are   often 

defined  as  the  units  of  all  plant  and  annual  life.  Plants  need 
phosphate,  nitrates,  etc.,  because  the  cells  must  have  them.  All 
the  problems  of  the  plant  relating  to  the  soil,  light,  temperature, 
etc.,  are  problems  of  the  cell.  The  plant  is  made  up  of  a  countless 
number  of  cells  and  the  activities  of  the  plant  are  simply  the  sum 
of  the  activities  of  the  many  cells  of  which  the  plant  is  composed. 
Discovery  of  the  Cell  and  Its  Structures.  —  That  plants  and 
animals  are  composed  of  cells  was  not  revealed  until  the  inven- 
tion of  the  microscope,   which,  although  very  rude  in   its  con- 

112 


PROTOPLASM  113 

struction  and  efficiency  as  compared  with  microscopes  of  today, 
was  beginning  to  be  employed  by  Robert  Hooke  (1635-1703) 
and  others  in  the  seventeenth  century  in  the  study  of  plants. 
Robert  Hooke,  one  of  the  earliest  to  study  plants  with  the 
microscope,  examined  thin  sections  of  cork,  and  found  the  cork 
to  be  composed  of  numerous  small  compartments  which  he 
called  cells  on  account  of  their  rough  resemblance  to  the  cells 
of  a  honeycomb.  Of  course  in  dead  tissue  like  cork  the  cell 
contents  are  absent,  and  Robert  Hooke  saw  only  the  cell  walls 
enclosing  the  spaces  from  which  the  active  substance  of  the 
cells  had  departed.  However,  through  investigations  which  fol- 
lowed those  of  these  earliest  investigators  with  the  microscope, 
it  gradually  came  to  be  recognized  that  the  important  part  of 
the  cell  is  the  substance  which  fills  the  compartments.  By  ex- 
tending the  study  to  many  kinds  of  tissues  of  both  plants  and 
animals,  it  was  finally  recognized  that  the  substance  filling  the 
cell  is  the  only  living  substance  in  plants  and  animals  and  the 
substance  which  builds  the  cell  wall  and  the  entire  organism. 
Various  names  were  at  first  applied  to  this  substance  before  the 
term  protoplasm  suggested  by  Hugo  Von  Mohl  was  adopted  by 
both  botanists  and  zoologists.  As  the  word  protoplasm,  which 
is  a  combination  of  protos  (first)  and  plasma  (thing  formed),  sig- 
nifies, this  substance  was  considered  the  first  organic  substance 
formed  from  the  inorganic  materials  taken  into  the  plant.  The 
idea  that  the  protoplasm  is  the  essential  substance  and  that  the 
cell  is  the  unit  of  plant  and  animal  structure  was  quite  thoroughly 
elaborated  by  Schleiden  (1838)  and  Schwann  (1839)  and  became 
generally  accepted. 

Protoplasm.  —  The  protoplasm,  as  already  noted,  is  the  living 
substance  of  plants  and  animals.  The  protoplasm  of  an  indi- 
vidual cell  is  often  called  a  protoplast.  Protoplasm  is  a  fluid 
substance  which  varies  much  in  its  consistency,  sometimes  being 
a  thin  viscous  fluid  like  the  white  of  an  egg,  and  sometimes  being 
more  dense  and  compactly  organized.  Chemical  analyses  show 
that  protoplasm  has  the  composition  of  protein,  although  such 
analyses  necessarily  kill  the  protoplasm  and  consequently  do  not 
give  us  a  true  knowledge  of  the  protoplasm  as  it  is  while  living. 
Although  the  protoplasm  of  higher  plants  usually  exhibits  no 
motion  except  when  dividing,  there  are  cases,  however,  as  in  the 
hairs  of  the  Pumpkin  and  Wandering  Jew,  where  the  protoplasm, 


114 


CELLS  AND   TISSUES 


when  under  the  microscope,  can  be  seen  streaming  around  the  cell 
wall  or  across  the  cell  from  side  to  side  or  end  to  end. 

The  protoplasm  consists  of  a  number  of  structures  which  differ  in 
organization,  and  each  of  which  has  one  or  more  special  functions. 
(Fig.  106.)  One  of  the  most  conspicuous  of  these  structures  is  the 
nucleus,  which  is  a  comparatively  compact  protoplasmic  body, 
usually  spherical  in  shape.  Although  usually  centrally  located 
in  actively  growing  cells,  the  nucleus  commonly  has  a  lateral  posi- 
tion in  old  cells.     The  nucleus  is  enclosed  by  a  membrane,  called 

the  nuclear  membrane,  and  is 
filled  with  a  hquid  known  as 
nuclear  sap,  which  consists  of 
water  and  dissolved  substances. 
However,  nuclear  sap  is  usually 
colorless  and,  therefore,  not  vis- 
ible. Within  the  nucleus  also 
occur  one  or  more  small  globu- 
lar bodies  known  as  nucleoli 
(singular  nucleolus)  and  much 
chunky  or  granular  material 
known  as  chromatin,  which  is 
regarded  as  the  most  impor- 
tant part  of  the  nucleus  and  is 
so  named  because  it  stains  so 
readily  when  stains  are  applied 
to  the  cell.  Around  the  nu- 
cleus and  filling  up  the  general 
cavity  within  the  cell  wall  is 
that  portion  of  the  protoplasm, 
known  as  cytoplasm,  which  is  a  loose  spongy  structure  full  of  many 
cavities  called  vacuoles.  The  vacuoles  are  filled  with  a  liquid  called 
cell  sap,  which  hke  the  nuclear  sap  consists  of  water  containing 
dissolved  sugars,  salts,  and  other  substances.  The  border  of 
the  cytoplasm  is  in  contact  with  the  cell  wall  and  is  modified 
into  a  membrane  known  as  the  cell  membrane,  which,  since  it  is 
closely  applied  to  the  cell  wall,  can  not  ordinarily  be  seen  until 
the  cell  is  bathed  in  salt  water  or  some  other  solution  strong 
enough  to  shrink  the  protoplasm,  so  that  the  cell  membrane  is 
drawn  away  from  the  wall  where  it  can  be  seen.  {Fig.  107.) 
Within  the  cytoplasm  commonly  occur  a  number  of  small  bodies 


Fig.  106. — -A  growing  cell,  iv,  cell 
wall;  c,  cytoplasm;  v,  vacuoles  filled 
with  cell  sap;  n,  nucleus;  a,  nucleoli; 
m,  nuclear  membrane;  g,  chromatin 
granules.  Enlarged  about  five  hun- 
dred times. 


CELL  WALL 


115 


Fig.  107.  —  Cells  with  protoplasm 
(p)  shrunken  to  show  the  cell  mem- 
brane, which  is  represented  by  the 
dotted  line  surrounding  the  proto- 
plasm. 


known  as  plastids,  which  are  masses  of  cytoplasm  but  denser  than 
ordinary  cytoplasm.  (Fig.  108.)  They  often  develop  pigments 
as  in  case  of  leaves,  stems,  and  other  green  organs  where  they 
develop  chlorophyll,  the  pigment  upon  which  the  green  color  of 
these  organs  depends.  Plastids 
containing  chlorophyll  are  called 
chloroplasts  and  are  very  impor- 
tant structures  because  they  have 
so  much  to  do  with  making  plant 
food.  Plastids  which  occur  in 
the  petals  of  some  flowers  have 
yellow  or  red  pigments.  Plastids 
which  are  colorless,  having  no 
pigments  at  all,  are  called  leuco- 
plasts.  Starch  grains  and  other 
small  bodies  (chondriosomes)  not 
shown  in  our  figure  are  also  commonly  present  in  the  cytoplasm. 
Cell  Wall.  —  The  cell  wall  is  formed  by  the  protoplasm  and 
may  be  variously  modified  by  it.  In  actively  growing  cells  the 
wall  is  thin  and  composed  of  cellulose  —  a  substance  which  allows 
the  wall  to  stretch  as  the  protoplasm  ex- 
pands in  growth.  As  the  cell  develops,  the 
protoplasm  in  many  cases  thickens  the  cell 
wall  by  depositing  new  layers  of  material, 
which  may  be  of  cellulose  or  of  some  other 
substance  better  adapted  to  the  function 
which  the  cell  is  to  perform.  In  nearly  all 
plants  but  in  trees  more  especially  some 
cells  deposit  lignin  in  their  walls,  thus  be- 
coming the  wood  cells  which  give  rigidity 
to  the  plant  and  which  we  use  in  the  form 
of  lumber.  In  the  bark  of  trees.  Potato 
skins,  and  other  structures  for  protection, 
fat-like  substances  are  deposited  in  the 
walls  of  the  cells  which  then  are  known  as 
cork.  Sometimes,  as  in  the  so-called  hast  fibers,  which  are  the 
strengthening  fibers  especially  prominent  in  Flax  and  Hemp,  the 
walls  are  extremely  thickened  with  cellulose.  The  same  is  true 
in  Date  seeds  and  Ivory  Nuts  where  the  walls  are  extremely 
thickened  with  cellulose  to  be  used  as  a  food  during  germination. 


^^ch 


Fig.  108.— Cell  from 
a  leaf,  lo,  cell  wall; 
n,  nucleus;  v,  a  large 
vacuole  in  the  cyto- 
plasm; ch,  chloroplasts. 


116 


CELLS   AND   TISSUES 


There  are  many  ways  in  which  cell  walls  are  modified  as  will  be 
seen  in  the  study  of  tissues. 

Processes  Involved  in  Cell  Activity.  —  The  chief  of  cell  activi- 
ties is  growth  which  will  be  discussed  in  connection  with  the  differ- 
ent plant  organs.  But  growth,  besides  being  much  under  the 
influence  of  external  conditions,  such  as  temperature  and  light, 
depends  upon  metabolism  —  the  proc- 
ess by  which  materials  are  changed 
into  forms  which  have  to  do  with 
growth.  In  connection  with  growth, 
metabolism,  and  other  physiological 
processes  of  the  cell,  osmosis  and  res- 
piration are  involved,  both  of  which 
were  shown  to  be  important  proc- 
esses in  seed  germination.  In  germi- 
nation and  other  physiological  proc- 
esses they  are  important  because  of 
their  connection  with  the  other  proc- 
esses of  cells.  Osmosis  is  a  physical 
process  which  occurs  wherever  two 
liquids  differing  in  concentration  are 
separated  by  a  membrane  which  they 
wet,  and  hence  is  not  a  cell  activity 
except  in  so  far  as  the  protoplasm 
controls  it  when  occurring  in  connec- 
.tion  with  the  cell.  Respiration,  on 
the  other  hand,  is  a  physiological 
process  and  only  occurs  in  connec- 
tion with  protoplasm. 

Osmosis.  —  Osmosis  may  be  de- 
fined as  that  kind  of  diffusion  by 
which  liquids  pass  through  mem- 
branes and  its  principles  can  be  best 
understood  by  the  study  of  an  illustration  as  shown  in  Figure 
109.  Thus  if  a  pig's  bladder,  filled  with  a  sugar  solution  and 
having  a  long  glass  tube  fastened  in  its  neck,  is  submerged  in 
a  jar  of  water,  water  will  pass  in  and  force  the  solution  up  the 
glass  tube.  If,  on  the  other  hand,  a  sugar  or  salt  solution  stronger 
than  the  one  in  the  bladder  be  placed  in  the  jar,  the  water  slowly 
passes  out  of  the  bladder.     Thus  the  water  passes  from  the  weaker 


Fig.  109. —  Experiment  dem- 
onstrating osmosis.  The  pig's 
bladder  was  filled  with  a  sugar 
solution  and  then  the  tube  was 
attached.  The  water  from  the 
jar  was  drawn  into  the  bladder 
and  the  solution  in  the  bladder 
forced  up  the  tube. 


OSMOSIS  117 

solution  through  the  nieiul^rane  to  the  stronger.  Sometimes 
some  of  the  dissolved  substances  may  pass  through  the  membrane, 
but  often  the  membrane  permits  only  the  water  to  pass,  in  which 
case  it  is  known  as  a  semi-permeahle  membrane.  In  case  of  the 
pig's  bladder  not  much  sugar  is  allowed  to  pass  through  its  wall, 
which  is,  therefore,  semi-permeable  in  reference  to  this  particular 
solution.  When  a  membrane  will  allow  a  dissolved  substance 
to  pass,  it  is  said  to  be  permeable  to  that  substance.  Most 
membranes  are  permeable  to  some  substances  and  impermeable 
to  others. 

The  causes  of  the  movement  of  the  water  or  other  solvents  from 
the  less  dense  to  the  denser  solution  are  not  thoroughly  under- 
stood. Some  think  that  it  is  due  to  the  affinity  of  the  dissolved 
substances  for  the  solvent,  which  is  pulled  to  the  substances  with 
a  force  increasing  with  the  amount  of  the  substances  in  solution. 
Others  think  that  it  is  due  to  the  checking  of  the  diffusive  power 
of  the  dissolved  substances  by  the  membrane,  which  permits  the 
two  liquids  to  approach  an  equilibrium  only  through  the  passing 
of  more  of  the  solvent  to  the  denser  solution. 

In  comparing  osmosis  in  the  cell  with  the  illustration,  the  ceU 
membrane  corresponds  to  the  wall  of  the  pig's  bladder,  the  cell 
sap  to  the  solution  within  the  bladder,  and  the  solutions  around 
the  cell  correspond  to  the  water  or  solutions  in  the  jar.  If  the 
cell  sap  in  denser  than  the  solution  on  the  outside  of  the  cell  mem- 
brane, then  water  with  those  dissolved  substances  to  which  the 
membrane  is  permeable  will  pass  in;  but,  on  the  other  hand,  if  the 
cell  sap  is  less  dense  than  the  solution  without,  water  and  prob- 
ably some  dissolved  substances  will  pass  out.  Thus  the  passing 
of  liquids  through  the  cell  membrane  from  a  less  dense  to  a  denser 
liquid  is  also  the  chief  feature  of  osmosis  in  cells.  It  should  also 
be  noted  in  connection  with  osmosis  in  cells:  (1)  that  the  more 
the  two  solutions  separated  by  the  cell  membrane  differ  in  con- 
centration, the  more  rapid  is  the  process  of  osmosis;  and  (2)  that 
the  solvent,  which  is  water  in  case  of  cells,  passes  through  the 
membrane  independently  of  its  dissolved  substances,  which  are 
either  carried  along  or  left  behind  according  to  whether  or  not 
the  membrane  is  permeable  to  them. 

However,  it  is  only  in  principle  and  not  in  practice  that  osmosis 
as  demonstrated  with  the  pig's  bladder  is  identical  with  that  in 
the  cell.     In  the  first  place,  instead  of  a  solution  containing  only 


118  CELLS   AND   TLSSUES 

one  dissolved  substance,  both  the  cell  sap  and  the  solution 
around  the  cell  usually  carry  in  solution  a  number  of  substances, 
each  of  which  in  its  osmotic  influence  is  independent  of  the  others, 
although  the  osmotic  influences  of  all  are  combined  in  determin- 
ing the  osmotic  force  of  the  solution.  In  the  second  place,  the  cell 
membrane  is  a  living  membrane  and,  therefore,  able  to  alter  its 
permeability,  so  that  it  may  be  permeable  to  certain  substances 
at  one  time  but  not  at  another.  Another  peculiar  feature  of  pro- 
toplasm is  that  substances  are  often  allowed  to  pass  in  more 
readily  than  out.  Thus  root  hairs,  which  take  in  many  sub- 
stances from  the  soil,  do  not  allow  the  sugars  and  many  other 
substances  in  their  cell  sap  to  pass  out.  If  the  cells  of  a  red  Beet 
are  laid  in  a  strong  salt  or  sugar  solution,  the  water  will  pass  out 
but  the  coloring  matter  will  be  retained.  Furthermore,  when 
some  cells  are  placed  in  very  dilute  solutions  of  dyes  as  methylene 
blue,  the  dye  accumulates  in  the  cell  sap,  which,  therefore,  be- 
comes much  more  colored  than  the  surrounding  solution.  In 
this  way  various  kinds  of  substances  which  are  allowed  to  pass  in 
more  readily  than  out  may  become  more  concentrated  in  the 
cell  sap  than  in  the  solution  without. 

It  is  now  seen  that  by  osmosis  cells  obtain  their  water  supply 
which  they  pull  from  the  soil,  surrounding  cells,  conductive  tracts, 
or  whatever  suiToundings  they  may  have  that  puts  them  in  con- 
tact with  water.  Furthermore,  the  more  concentrated  their  cell 
sap,  the  more  forcibly  and  rapidly  they  can  draw  water  from  their 
surroundings.  Osmosis,  although  chiefly  concerned  with  supply- 
ing cells  with  water,  assists  some  in  supplying  cells  with  dissolved 
minerals,  sugars,  and  other  substances,  which  the  cell  membrane 
permits  to  be  carried  in  with  the  water.  But  in  connection  with 
osmosis  substances  may  pass  into  and  out  of  cells  by  the  same 
principles  which  are  active  in  ordinary  diffusion.  Thus  if  sub- 
stances are  less  concentrated  in  the  cell  sap  than  without  and  the 
membrane  is  permeable  to  them,  they  will  diffuse  to  the  cell  sap, 
more  or  less  independently  of  the  movement  of  water,  although  if 
the  water  is  moving  in  the  same  direction  the  substances  will 
move  more  rapidly.  Likewise  substances  diffuse  out  of  cells 
when  more  concentrated  within  than  without,  provided  the  cell 
membrane  is  permealile  to  them. 

Pressure  Within  the  Cell.  —  In  the  case  of  the  pig's  bladder, 
it  is  seen  that  the  flow  of  water  into  the  interior  increases  the 


CHARACTER  OF   THE   CELL   MEMBRANE  AFTER   DEATH      119 

amount  of  solution  within  until  sonic  of  the  solution  is  forced 
up  the  tube.  The  solution  rises  in  the  tube  because  the  increase 
in  the  amount  of  solution  within  the  bladder  requires  more  space, 
and  is,  therefore,  accompanied  by  an  increase  in  pressure  against 
the  wall  of  the  bladder.  This  pressure,  which  in  this  case  de- 
pends upon  the  concentration  of  the  sugar  solution  in  the  bladder, 
might  become  so  great  as  to  burst  the  bladder,  if  no  tube  for  an 
outlet  were  provided.  This  pressure,  known  as  osmotic  pressure, 
has  been  found  to  follow  quite  well  the  laws  governing  gas  pres- 
sure. Consequently,  if  the  number  of  molecules  of  the  dissolved 
substance  contained  in  a  certain  volume  of  the  solution  is  known, 
the  osmotic  pressure  can  be  calculated.  Thus  342  grams  of  Cane 
sugar  in  1  liter  of  solution  (called  a  gram-molecular  solution)  will 
exert  a  pressure  of  about  22.3  atmospheres  or  336  lbs.  and  in 
whatever  proportion  the  number  of  grams  is  increased  or  de- 
creased, the  pressure  is  altered  in  a  similar  proportion.  Osmotic 
pressure  in  cells,  where  it  is  called  turgor  pressure,  is  usually  not 
less  than  50  lbs.  and  often  more  than  100  lbs.  The  rigidity  of 
organs  such  as  leaves,  soft  stems,  and  roots  is  largely  due  to  turgor 
pressure,  as  can  be  easily  shown  by  immersing  strips  of  a  fresh 
Beet  or  Radish  in  a  strong  solution  where  they  lose  water  and 
become  flaccid.  The  wilting  in  leaves  when  exposed  to  excessive 
evaporation  is  due  to  the  loss  of  turgor  pressure,  which  occurs 
whenever  cells  lose  water  more  rapidly  than  they  absorb  it.  The 
preservative  value  of  such  substances  as  salts  and  sugars  when 
applied  to  meats  and  fruits  depends  largely  upon  the  withdrawal 
of  water  from  the  micro-organism,  so  that  they  can  not  become 
active.  Wilted  cells,  if  not  dead,  will  also  draw  in  water  and  again 
become  turgid  when  put  in  contact  with  moisture.  In  this  way 
flowers  are  revived  by  placing  their  stems  in  water,  and  Cucum- 
bers, Lettuce,  and  Celery  are  made  crisp  by  putting  them  in  cold 
water.  Sometimes,  as  the  pollen  of  some  plants  illustrates  when 
immersed  in  water,  the  pressure  becomes  so  great  that  the  cells 
burst.  Even  fruits,  such  as  Plums,  sometimes  burst  on  the  trees 
from  this  cause  when  the  weather  is  warm  and  moist. 

The  Character  of  the  Cell  Membrane  After  Death.  —  With  the 
death  of  the  cell,  the  cell  membrane  ceases  to  be  an  osmotic  mem- 
brane and  thus  becomes  permeable  to  all  substances  in  solution. 
After  the  cell  membrane  is  dead  substances  pass  through  it, 
either  into  or  out  of  the  cell,  almost  as  easily  as  through  a  piece  of 


120  CELLS  AND  TISSUES 

cloth.  Consequently,  osmotic  pressure  is  lost  when  cells  die  and 
the  substances  ordinarily  retained  are  allowed  to  diffuse  out. 
This  is  easily  demonstrated  by  soaking  plant  tissues  in  water  be- 
fore and  after  death.  Thus,  if  from  a  fresh  red  Beet  a  strip  is  cut, 
washed  thoroughly  so  as  to  remove  the  contents  of  the  injured 
cells,  and  then  soaked  in  water  at  a  temperature  not  destructive 
to  the  life  of  the  cell,  it  will  be  found  that  the  pigment,  sugar,  and 
other  substances  of  the  cell  are  retained;  but  if  the  strips  are  put 
in  water  hot  enough  to  kill  the  cells,  then  the  pigment,  sugar,  and 
other  cell  substances  diffuse  out  into  the  water.  That  pools  in 
which  dead  leaves  fall  soon  become  colored  is  a  common  observa- 
tion. The  fact  has  significance  for  the  farmer  who  has  learned 
by  experience  that,  when  hay  that  is  down  is  caught  in  a  rain, 
more  of  the  elements  are  washed  from  the  cured  hay  than  from 
that  more  recently  mowed  and  hence  still  partly  green. 

Nature  of  Plant  Food.  —  Besides  oxygen,  which  is  chiefly  used 
in  respiration,  various  substances,  such  as  water,  sugar,  acids, 
salts,  and  carbon  dioxide,  enter  the  protoplasm  where  most  of 
them  have  some  use  related  to  the  growth  of  the  plant.  But  as  to 
whether  or  not  all  should  be  considered  as  plant  foods,  not  all 
students  of  plants  agree;  for,  although  all  of  these  substances 
have  to  undergo  transformations  in  becoming  cell  structures, 
some  are  more  nearly  ready  for  use  than  others.  This  may  be 
illustrated  by  comparing  sugar  with  carbon  dioxide  and  water. 
In  the  leaves  or  wherever  chlorophyll  is  present,  carbon  dioxide 
and  water  have  their  elements  dissociated  and  combined  in  such 
a  way  as  to  form  sugar  which  can  be  used  directly  for  respiration 
or  by  minor  chemical  changes  be  transformed  into  cell  walls. 
Thus  sugar,  since  it  is  more  nearly  ready  for  use,  may  be  called  a 
food  and  the  carbon  dioxide  and  water  may  be  called  elements  from 
which  food  is  made.  Likewise  protein,  which  is  closely  related  to 
protoplasm,  may  be  regarded  as  a  food,  while  the  mineral  salts, 
such  as  nitrates,  phosphates,  sulfates,  etc.,  which  are  necessary  in 
the  formation  of  proteins,  may  be  egarded  as  the  elements  from 
which  food  is  made.  Some  investigators  restrict  the  term  plant 
food  to  the  more  complex  substances,  such  as  sugars,  starch,  pro- 
teins, fats,  and  amino  acids,  while  others  include  some  of  the  sim- 
pler elements,  especially  the  mineral  salts.  In  this  presentation 
water,  carbon  dioxide,  and  the  mineral  salts  are  regarded  as  ele- 
ments used  in  the  formation  of  foods, 


^     ,  ^      '      .  ^  RESPIRATION  121 

^  K  •    ^ 

Y^  y-   )     ^  Respiration 

The  general  features  of  respiration  were  discussed  in  connec- 
tion with  seed  germination  where  respiration  is  not  only  promi- 
nent but  also  must  be  reckoned  with  in  understanding  the  ger- 
minative  process.  There  it  was  stated  that  respiration  takes 
place  only  within  the  cell  and  that  it  is  comparable  to  ordinary 
combustion  in  that  it  is  an  oxidation  process  resulting  in  the 
breaking  down  of  substances  into  simpler  elements  with  the  re- 
lease of  potential  energy. 

It  is  a  well  known  fact  that  whenever  carbon  and  oxygen  are 
united  energy  is  released.  This  is  the  principle  employed  in  heat- 
ing plants,  steam  engines,  etc.  where  energy  in  the  form  of  heat  is 
obtained  through  the  union  of  oxygen  with  the  carbon  in  the  coal, 
wood,  or  some  other  combustible  substance.  If  sugars,  starches 
or  other  substances  containing  carbon  were  used  for  fuel,  the  same 
results  would  be  obtained.  In  the  cell,  however,  since  most  of 
the  energy  released  is  used  in  protoplasmic  movements,  and  in 
chemical  changes  involved  in  enlarging  cell  walls,  making  more 
protoplasm,  etc.,  not  much  is  exhibited  as  heat,  although  enough 
that  all  living  plant  parts  are  generally  a  little  warmer  than  their 
surroundings,  sometimes  2  or  3  degrees  in  case  of  large  flowers 
and  often  much  more  in  germinating  seeds.  Again,  although 
the  rate  of  respiration  increases  with  the  temperature  up  to  a 
certain  point,  respiration  proceeds  in  a  lower  temperature  than 
does  ordinary  combustion.  In  fact,  a  temperature  high  enough 
to  start  the  combustion  of  most  substances  is  entirely  too  high 
for  respiration,  which  in  most  plants  ceases  before  60°  C.  is 
reached.  Also  in  respiration  the  process  of  oxidation  is  initiated 
and  kept  going  by  enzymes  or  directly  by  the  protoplasm,  while 
there  are  no  such  agents  involved  in  comlmstion.  Thus,  although 
similar  in  results,  in  operation  respiration  is  very  different  from 
combustion. 

In  combustion  there  is  a  constant  ratio  between  the  oxygen 
used  and  the  carbon  dioxide  produced.  Thus  in  the  combustion 
of  Grape  sugar,  as  illustrated  by  the  formula  CeHioOe  +  6  Oo  = 

6CO2-I-6H2O,  the  ratio -T-7v^  is  1.     In  respiration,  however, 
0  U2 

although  the  ratio  is  often  unity,  it  varies  nuich,  sometimes  ])eing 

greater  and  sometimes  much  less  than  unity.     In  germinating 


122  CELLS  AND  TISSUES 

seeds,  tubers,  and  bulbs  containing  starch  and  sugars,  and  in 
many  other  plant  structures,  the  volume  of  oxygen  consumed 
during  active  respii'ation  is  equal  to  that  of  the  carbon  dioxide 
given  off;  but  in  the  germination  of  seeds  containing  fats  and 
fatty  oils,  the  volume  of  oxygen  consumed  is  greater  than  that  of 
the  carbon  dioxide  given  off,  in  which  case  some  of  the  oxygen 
is  apparently  used  in  changing  the  fats  and  fatty  oils  to  other 
forms  of  food  having  a  larger  proportion  of  oxygen. 

In  higher  plants  the  substances  oxidized  are  organic  compounds 
including  the  sugars,  fats,  proteins,  organic  acids,  and  probably 
the  protoplasm  itself.  Some  lower  forms  of  organisms  oxidize 
inorganic  compounds.  Some  Bacteria  obtain  energy  by  oxidizing 
the  ammonia  of  ammonia  salts  to  nitrites,  while  others  obtain 
energy  by  oxidizing  the  nitrites  to  nitrates.  Various  other  sub- 
stances, such  as  hydrogen  sulphide  and  iron,  are  oxidized  by 
certain  Bacteria  to  secure  energy. 

There  are  some  forms  of  respiration  which  can  continue  when 
oxygen  is  excluded  and  the  one  of  them  best  known  is  fermen- 
tation, which  is  prominent  in  the  Yeast  Plant  and  other  fer- 
menting organisms.  When  proceeding  in  the  absence  of  oxygen, 
such  forms  of  respiration  are  known  as  anaerobic  respiration,  that 
is,  respiration  in  the  absence  of  air.  In  fact,  some  micro-organ- 
isms can  not  carry  on  their  processes  well  except  in  the  absence  of 
air.  One  kind  of  anaerobic  respiration,  which  is  very  similar  to 
if  not  identical  with  fermentation,  can  be  detected  in  seeds,  fruits, 
and  all  living  plant  parts  when  oxygen  is  excluded,  so  that  the 
process  is  not  obscured  by  ordinary  respiration.  This  kind  of 
respiration  is  considered  by  some  to  be  the  initial  stage  of  ordinary 
respiration,  thus  being  closely  related  to  it. 

The  peculiar  feature  about  fermentation  in  the  absence  of  air 
is  that  oxidation  of  carbon  continues  with  the  release  of  energy 
and  the  production  of  carbon  dioxide,  although  no  oxygen  is  ob- 
tainable from  without.  Furthermore,  fermentation,  whether  in 
the  presence  or  absence  of  air,  differs  from  combustion  and  ordi- 
nary respiration  in  the  completeness  with  which  the  substances 
involved  are  broken  down.  This  may  be  illustrated  in  the  case 
of  the  fermentation  of  sugar  by  Yeast,  in  which  case,  as  shown  by 
the  equation  C6H12O6  =  2  COo  +  2  CoHeO,  the  molecule  of  sugar 
is  broken  into  2  molecules  of  carbon  dioxide  and  2  of  alcohol, 
while  in  case  of  combustion  and  often  in  respiration  the  molecule 


CELL   MULTIPLICATION  123 

of  sugar  is  broken  into  carbon  dioxide  and  water,  as  shown  in  the 
equation  CeHiaOe  +  6  O2  =  6  CO2  +  6  H2O.  In  respiration  the 
breaking  of  the  sugar  into  carbon  dioxide  and  alcohol  is  probably 
the  first  step  which  is  then  followed  by  the  breaking  of  the  alcohol 
into  carbon  dioxide  and  water.  From  the  equation  in  case  of  the 
fermentation  of  sugar  it  is  seen  that  the  energy  is  obtained  by 
uniting  the  oxygen  and  carbon,  both  of  which  are  present  in  the 
molecule  of  sugar.  Thus  by  the  use  of  the  oxygen  within  the 
compound  broken  down,  some  oxidation  can  occur  when  there  is 
no  oxygen  available  from  without. 

Instead  of  alcohol  other  substances  may  be  produced  by  fer- 
mentation according  to  the  nature  of  the  fermenting  organism 
and  the  kind  of  compound  fermented.  Thus  in  the  fermentation 
of  cider  by  certain  kinds  of  Bacteria  alcohol  is  first  produced  and 
later  acetic  acid.  In  the  souring  of  milk  the  Bacteria  break  the 
milk  sugar  into  lactic  acid.  Although  sugars  are  the  substances 
involved  most  in  fermentation,  other  compounds  are  known  to  be 
involved.  Even  decay,  caused  principally  by  Molds  and  Bac- 
teria, is  regarded  as  a  kind  of  fermentation,  in  which  case  many 
kinds  of  substances  are  involved. 

The  injury  caused  by  Fungi  and  Bacteria  is  often  due  largely  to 
the  by-products  of  their  respiration  and  growth.  Partly  in  this 
way  Fungi  damage  or  destroy  plants  upon  which  they  Uve.  Many 
of  the  Bacteria  associated  with  diseases  produce  poisons  known 
as  toxins  which  cause  injury  or  death  in  animals  and  sometimes 
in  plants.     To  combat  some  of  these  toxins  antitoxins  are  used. 

Thus  respiration  whether  aerobic  or  anaerobic  is  that  oxidation 
process  by  which  cells  secure  energy  to  carry  on  their  work.  Any 
condition ,  such  as  a  low  or  high  temperature,  absence  of  food,  or 
lack  of  oxygen,  which  hinders  respiration,  holds  cell  activity  in 
check  and  thus  impedes  plant  growth.  Furthermore,  due  to  the 
liberation  of  heat  and  moisture  which  may  become  destructive 
when  allowed  to  accumulate,  respiration  must  be  reckoned  with 
in  the  storing  of  plant  products. 

Cell  Multiplication 

As  previously  stated  (page  112),  cells  are  exceedingly  small 
structures  and  a  small  size  seems  preferable  in  both  plants  and 
animals  where  numerous  small  cells  rather  than  a  few  large  ones 


124  CELLS   AND  TISSUES 

is  the  rule.  Consequently,  as  a  cell  grows,  a  size  is  soon  attained 
at  which  division  must  occur.  By  division  the  cell  becomes  two 
cells  of  half  the  parent  size,  and  each  of  the  new  cells  has  all  of  the 
structures  of  the  parent  cell  and  the  ability  to  repeat  the  proc- 
esses of  growth  and  division. 

It  is  by  the  growth,  division,  and  differentiation  of  cells  that 
both  plants  and  animals  become  adult  individuals.  In  the  ferti- 
Hzed  egg,  the  first  stage  of  an  individual's  existence,  cell  division 
begins  usually  in  a  few  hours  after  fertilization  and  continues 
throughout  the  life  of  the  plant,  although  interrupted  at  various 
times.  Although  the  cell  divisions  are  countless  in  number  in  the 
higher  plants,  they  all  proceed  in  the  same  way  throughout  the 
plant,  except  in  the  anther  and  ovary  where  a  peculiar  type  of 
division  to  be  discussed  later  occurs. 

In  some  simple  plants,  as  Bacteria  and  the  Yeast  Plant  where 
cell  division  is  of  a  simple  type,  the  processes  of  division  may  oc- 
cupy only  a  few  minutes,  but  in  the  higher  plants  where  cell  divi- 
sion is  more  complex,  the  processes  of  division  often  require  two 
or  more  hours,  and  so  far  as  we  know  the  processes  are  continuous 
throughout  the  entire  period.  Most  of  this  time  is  occupied  by 
the  division  of  the  chromatin  about  which  cell  division  centers. 

Although  cell  division  consists  of  a  continuous  series  of  events, 
a  few  stages  in  the  process,  as  shown  in  Figure  110,  will  suffice  to 
give  an  understanding  of  cell  division  as  it  occurs  in  the  liigher 
plants.  Thus  starting  with  the  chromatin  in  a  granular  condi- 
tion and  scattered  through  the  nucleus,  the  first  step  in  division 
is  the  organization  of  this  chromatin  into  a  thread  which  then  is 
segmented  into  segments  known  as  chromosomes.  The  number 
of  chromosomes  into  which  the  thread  segments  is  definite  for 
each  plant  or  animal,  although  varying  much  in  different  species, 
ranging  from  two  in  some  worms  to  more  than  one  hundred  in 
some  Ferns.  However,  in  many  of  our  common  plants  and  ani- 
mals the  number  ranges  from  sixteen  to  forty-eight.  In  man 
there  are  forty-six  or  forty-eight,  in  Tomatoes  twenty-four,  and 
in  Wheat  sixteen.  The  chromosomes,  which  have  no  definite 
arrangement  when  first  formed,  soon  arrange  themselves  in  a 
plane  across  the  cell.  As  they  assume  this  arrangement,  the 
nuclear  membrane  disappears,  thus  allowing  the  chromosomes  to 
come  m  contact  with  the  fibers,  known  as  spindle  fibers,  which 
seem  to  be  special  provisions  of  the  cytoplasm  for  bringing  about 


CELL   MULTIPLICATION 


125 


the  distribution  of  the  chromosomes.  At  this  stage  it  becomes 
apparent  that  each  chromosome  consists  of  two  pieces  or  halves 
each  apparently  having  split  longitudinally.  The  halves  of  each 
chromosome  now  separate,  pass  to  opposite  ends  of  the  cell  where 
the  new  nuclei  are  formed.  Thus  each  new  nucleus  gets  as  many 
halves,  which  soon  grow  to  full  size  chromosomes,  as  there  were 
chromosomes  in  the  parent  cell.     As  the  new  nuclei  are  forming 


b 

c 

PvT":^ 

r  V 

m 

:^ 

^ 

"M , 

I:  /:m 

yiVi 

e  f  g  h 

Fig.  110.  —  Cell  division,  o,  cell  in  resting  stage,  h,  chromatin  formed 
into  a  thread,  c,  the  thread  of  chromatin  broken  into  segments  called  chromo- 
somes, d,  chromosomes  arranged  across  the  cell  for  division.  Notice  the 
threads  called  spindle  fibers  running  through  the  cell  and  that  the  nuclear 
membrane  has  disappeared,  c,  chromosomes  have  split  and  the  halves  are 
passing  to  opposite  ends  of  the  cell.  /,  chromosomes  have  reached  the  points 
where  they  are  to  form  new  nuclei,  g  and  h,  new  nuclei  and  cross  wall  be- 
tween them  forming.     After  Stevens. 


a  cross  wall  is  formed,  which  divides  the  cytoplasm,  and  cell  divi- 
sion is  now  complete.  Instead  of  one  cell  there  are  now  two, 
each  of  which  after  growing  to  full  size  will  divide  in  the  same 
manner  as  the  parent  cell. 

Except  in  certain  regions  where  cell  multiplication  is  the  spe- 
cial function,  most  cells  of  the  plant  sooner  or  later  lose  their 
ability  to  grow  and  divide  as  a  result  of  their  modifications 
which  adapt  them  to  their  special  functions.     Thus  after  cells  are 


126 


CELLS  AND   TLSSUES 


thoroughly  modified  for  protection,  absorption,  strength,  conduc- 
tion, food-making,  etc.,  in  most  cases  growth  and  division  ceases. 
This  brings  us  to  the  tissues  which  are  groups  of  cells  so  modified 
as  to  be  adapted  to  special  functions  and  upon  which  the  various 
activities  of  the  plant  depend. 

General  View  of  Tissues 

The  most  important  tissues  of  Seed  Plants  are  those  which  have 
to  do  with  growth,  protection,  support,  conduction,  secretions, 
absorption,  food  manufacture,  food  storage,  and  I'eproduction. 


Fig.  IIL  —  .1,  lengthwise  section  through  a  tip  of  a  stem,  showing  the 
apical  meristem  {m)  from  which  branches  (b)  are  arising  and  from  which  cam- 
bium (c)  and  other  tissues  are  being  formed  below.  B,  cross  section  of  a 
stem,  showing  the  cambium  and  its  position  in  reference  to  other  tissues. 


Tissues  Connected  with  Growth.  —  Since  the  cells  of  most  tis- 
sues are  no  longer  capable  of  growth  and  division  after  completing 
their  modifications,  there  must  be  provided  at  certain  places  in 
the  plant  groups  or  bands  of  cells  which  retain  their  ability  to 
grow  and  divide  throughout  the  life  of  the  plant,  for  otherwise 
the  growth  of  the  plant  would  soon  cease.  Such  cells,  forming 
the  meristematic  tissues  or  meristems  (from  the  Greek  word  mean- 
ing "to  divide  ")>  are  present  at  the  stem  and  root  tips  and  in  the 
cambium,  where  their  chief  function  is  the  multiplication  of  cells, 


PROTECTIVE   TISSUES  127 

SO  that  the  different  tissues  may  be  enlarged  as  the  growth  of  the 
plant  demands.  By  means  of  a  meristem  at  their  tips,  roots 
and  stems  elongate,  and  by  means  of  the  cambium  they  in- 
crease in  circumference.  However,  the  meristems  at  the  growing 
apices  are  the  first  sources  of  all  other  tissues,  even  of  the  cam- 
bium, and  for  this  reason  are  known  as  the  primary  meristems. 
(Fig.  111.) 

Meristematic  cells  are  characterized  by  having  thin  cellulose 
walls,  large  nuclei,  and  dense  cytoplasm  —  features  which  enable 
the  cell  to  grow  and  divide  rapidly.  Closely  related  to  the  meri- 
stematic cells  are  the  parenchyma  cells,  which  also  in  most  cases 
have  thin  cellulose  walls  but  are  less  active  in  dividing.  Paren- 
chyma cells  occur  scattered  throughout  the  various  plant  tissues 
and  constitute  the  food-making  tissues  of  leaves  and  stems,  and 
most  of  the  pith  of  plants. 

Protective  Tissues.  —  For  protection  against  destructive  agen- 
cies plants  have  their  outer  cells  modified  into  protective  tissues, 


B 

Fig.  112.  —  A,  epidermis  of  a  leaf  showing  epidermal  cells  (e)  with  their 
outer  cutinized  walls  (c).  B,  the  flesh  (j)  and  rind  of  a  Jonathan  Apple  show- 
ing the  thick,  cutinized,  outer  walls  (c)  of  the  epidermal  cells  (e).  Much 
enlarged. 

such  as  epidermis,  corky  rind,  and  bark,  which  lessen  evaporation 
and  prevent  the  entrance  of  destructive  organisms.  The  most 
common  protective  tissue  is  the  epidermis  which  consists  of  one 
or  more  layers  of  cells  forming  a  jacket  about  the  plant.  The 
outer  walls  of  the  exterior  layer  of  epidermal  cells  are  usually 
thickened  and  contain  a  waxy  substance  called  cutin  which  makes 
them  waterproof.  (Fig.  112.)  Most  plant  organs  are  at  first  pro- 
tected by  an  epidermis,  but  in  the  older  portions  of  stems  and  roots 
the  epidermis  is  often  replaced  by  cork  tissue,  which  is  usually 


128 


CELLS  AMD  TISSUES 


inuc-h  thicker  and  more  protective  than  an  epidermis.  {Fig.  US.) 
The  cork  covering  ma\'  be  more  or  less  flexible,  as  the  rind  of  an 
Irish  Potato  or  Sweet  Potato,  or 
harder  and  more  brittle,  as  in  the 
bark  of  trees,  where  it  reaches  its 
extreme  thickness.  Cork  tissue  con- 
sists of  dead  cells  in  the  walls  of 
which  there  is  deposited  a  waxy 
substance  much  like  cutin  but  called 
suherin  to  which  much  of  the  pi'o- 
tective  character  of  cork  is  due. 
Cork  coverings  afford  more  protec- 
tion than  an  epidermis,  but  on  ac- 
count of  their  opaqueness,  they  are 
not  suitable  except  where  it  is  not 

necessary  for  light  to  penetrate  to 
Fig.   113. —  a  small   portion    ^,       •  ,• 

Irish    ^"^  inner  tissues. 


of   a    section  through  an 
Potato,     r,   rind  composed  of  a 


The  protection  afforded  by  an 
umber  of  layers  of  cork  cells,  epidermis  and  cork  is  often  brought 
s,  tissue  filled  with  food.  Highly  to  our  notice  in  case  of  fruits,  tubers, 
magnified.  ^^^^    fleshy    roots.      Thus    Apples, 

Oranges,  and  most  fruits  which  may  be  kept  a  long  time,  if 
uninjured,  soon  decay  when  their  rinds 
are  broken.  The  efficiency  of  a  corky 
rind  to  protect  against  the  loss  of  water 
is  shown  by  the  experiment  in  which  a 
peeled  Irish  Potato  lost  sixty  times  as 
much  water  in  48  hours  as  an  unpeeled 
one  of  equal  weight. 

Furthermore,  cork  tissue  has  an  ad- 
ditional function  in  the  healing  of 
wounds  where,  by  the  development  of 
a  callus-like  mass  of  cork,  the  open-  ^^^  ii4. -Some  coUen- 
ing  of  the  wound  is  closed  and  the  chyma  cells  from  the  stem 
break  in  the  protective  covering  of  of  a  Dock  (Rumex)  showing 
the  plant  thereby  repaired.  It  is  im-  the  cells  thickened  mainly 
portant  to  recognize  this  fact  in  prun-    at  the  angles.    After  Cham- 

,  ,  11         ii         berlam. 

mg  where  the  promptness  as  well  as  the 

thoroughness  of  the  healing  depends  much  upon  how  the  wound 
is  made. 


STRENGTHENING   TISSUES 


129 


Strengthening  Tissues.  ■ —  In  order  to  endure  the  strains  to 
which  they  are  exposed,  both  stems  and  roots  must  have  strength- 
ening tissues  so  as  to  be  tough  and  rigid.  Strengthening  cells, 
although  of  different  types,  have  much  thickened  walls  and  in 
most  cases  are  much  elongated. 

In  one  kind  of  strengthening  tissue,  known  as  collenchyma, 
which  often  occurs  in  the  younger  regions  of  stems,  the  cell 
walls  are  thickened  chiefly  at  the  angles,  thus  leaving  thin 
portions  in  the  side  walls  through  which  the  protoplasm  receives 
enough  materials  to  maintain  life  in  spite  of  the  modifications. 
{Fig.  114.) 

b 


Fig.  115.  —  Bast  fibers  of  Flax.  A,  a  portion  of  a  cross  section  of  a  Flax 
stem,  showing  the  bast  fibers,  e,  epidermis;  h,  bast  fibers;  w,  woody  part  of 
the  stem;  p,  pith.  B,  longitudinal  view  of  a  number  of  bast  fibers.  Much 
enlarged. 

A  kind  of  strengthening  tissue,  in  which  the  cell  walls  are  quite 
evenly  thickened  with  cellulose,  occurs  in  the  older  regions  of 
stems  between  the  epidermis  and  woody  cylinder,  and  consists 
of  hast  fibers,  the  fibers  upon  which  the  value  of  Flax,  Hemp,  etc. 
as  fiber  plants  depends.  Fig.  115.)  Bast  fibers  are  much  elon- 
gated cells  and  so  spliced  that  they  form  thread-like  fibers  which 
are  easily  combined  into  larger  fibers  for  making  linen  cloth, 
twine,  ropes,  and  other  textiles.  Bast  fibers  may  occur  also  in 
leaves  and  roots  where  they  are  usually  not  so  prominent,  how- 
ever, as  in  stems. 

In  the  woody  portions  of  plants,  especially  in  all  trees  except 
the  evergreens,  there  occur  along  with  the  conductive  tissues 
wood  fibers,  in  which  the  walls  of  the  much  elongated  cells  are  not 


130 


CELLS  AND   TLSSUES 


only  much  thifkened  l)ut  also  made  woody  —  a  feature  in  which 
they  differ  from  collenchyma  and  bast  fibers,  where  the  thicken- 
ings are  mainly  of  cellulose.  {Fig.  116.)  Where 
the  wood  fibers  are  abundant,  as  in  Oaks,  the  wood 
is  compact.  Likewise,  due  to  a  greater  numl^er  of 
wood  fibers,  fall  wood  is  more  compact  than  spring 
wood. 


Fig.  116.  — a 
wood  fiber,  con- 
sisting of  a  much 
elongated  cell 
with  thick 
woody  walls. 


Fig.  117.  —  Very  much  enlarged  lengthwise  section 
through  an  Alfalfa  stem,  showing  the  conductive  and  food- 
making  tissues  of  the  stem.  /,  tracheae  (commonly  called 
xylem),  which  constitute  the  water-conducting  tissue; 
■p,  the  conductive  tissue  (commonly  called  phloem),  which 
conducts  the  food  made  by  the  leaves;  c,  the  food-making 
and  storage  tissue  (cortex)  just  under  the  epidermis  (e). 
The  cells  of  the  cortex  contain  chloroplasts  {ch).  a, 
cambium. 


Conductive  Tissues.  —  The  conductive  tissues  of  plants  are  of 
two  kinds,  xylem  and  phloem,  which  occurring  together  form  the 
vascular  bundles  through  which  water,  mineral  salts,  and  foods 
are  distributed  to  all  parts  of  the  plant.  {Fig.  117.)  The  xylem 
is  devoted  chiefly  to  carrying  water  with  what  it  may  have  in 
solution  and  the  phloem  to  carrying  foods.  Furthermore,  the 
xylem  and  phloem  differ  in  that  the  conductive  cells  of  the  former 
are  empty  while  the  conductive  cells  of  the  latter  retain  their  pro- 
toplasm. In  Conifers,  such  as  Pines,  Firs,  etc.,  the  water-conduct- 
ing cells  have  tapering  ends  and  do  not  form  a  continuous  series. 
They  have  peculiar  pits  in  their  walls,  known  as  bordered  pits, 
through  which  the  liquids  pass  from  cell  to  cell.  They  are  com- 
monly known  as  tracheids,  meaning  "trachea-like."  Other  plants 
have  tracheids,  but  tracheids  with  bordered  pits  are  characteristic 
of  Conifers.     The  tracheids  are  also  important  strengthening  as 


ABSORBING   TISSUES 


131 


well  as  conductive  tissue.  {Fig.  118.)  In  Flowering  Plants, 
although  tracheids  are  present,  the  water-conducting  tissue  is 
composed  mainly  of  cells  which  fit 
together  end  to  end  and  thus  form  a 
continuous  series.  The  end  walls  of 
the  cells  of  the  series  are  resorbed  and 
thus  are  formed  continuous  tubes, 
called  ducts,  vessels,  or  tracheae,  the 
last  name  referring  to  their  resem- 
blance to  the  human  trachea.  In  the 
phloem,  the  main  conductive  tissue  is 
composed  of  the  sieve  tubes,  which  are 
so  named  because  of  the  perforations 
in  their  walls.  Unlike  tracheae,  which 
have  thickened  woody  areas  in  their 
walls,  sieve  tubes  have  thin  cellulose 
walls  and  retain  their  protoplasm. 
With  the  sieve  tubes  usually  occur 
thin-waUed  elongated  cells,  known  as 
companion  cells,  and  parenchyma  cells, 
both  of  which  aid  in  conduction. 

Absorbing  Tissues.  —  In  the  higher 
plants,  where  the  plant  body  is  dif- 
ferentiated into  roots,  stem,  and  leaves,  the  roots  are  especially 
devoted  to  absorption.     In  case  of  soil  roots,   the  root  hairs, 


Fig.  118.  —  Tracheids  from 
wood  of  Pine,  showing  the 
tapering  ends  and  the  bor- 
dered pits  (p).  After  Cham- 
berlain. 


Fig.  119.  —  A,  root  hairs,  the  absorptive  structures  of  roots,  as  they  appear 
in  a  surface  view  of  the  tip  of  a  root.  B,  cross  section  of  a  root,  showing  that 
the  root  hairs  {h)  are  projections  of  the  epidermal  cells  (e). 


132 


CELLS  AND   TLSSUES 


which  spread  into  the  soil  where  they  take  up  water  by  means 
of  osmosis,  are  the  chief  absorptive  structures.  {Fig.  119.)  There 
are  some  plants,  however,  which  live  on  other  plants,  in  which 
case  the  root  tissues  absorb  directly  from  the  tissues  with  which 
they  are  in  contact.  In  some  cases  the  leaves  absorb,  as  in 
the  Sundew  (Drosera),  Venus's  Flytrap  {Dioncea  muscipula), 
and  Pitcher  Plants  {Sarracenia) ,  where  the  eaves  are  especially 
constructed  for  catching  and  absorbing  insects. 

Food-making  Tissues.  —  The  principal  food-making  organs  are 
the  leaves  where  the  cells  are  provided  with  chloroplasts  and  so 
arranged  that  they  can  obtain  the  raw  materials  from  which  foods 


Fig. 


120.  —  Cross  section  of  the  leaf.    /,  food-making  tissue;  e,  epidermis; 
V,  cross  section  of  a  vein. 


are  made.  (Fig.  120.)  However,  food-manufacture  is  not  lim- 
ited to  leaves,  for  all  green  stems  have  just  under  their  epidermis 
a  band  of  green  cells,  known  as  the  cortex,  in  which  food  is  manu- 
factured as  long  as  light  and  air  are  not  excluded.     {Fig.  117.) 

Storage  Tissues.  —  Any  living  cell  usually  contains  some 
stored  food,  ]3ut  there  are  cells  which  have  food  storage  as  their 
chief  function.  This  is  true  in  the  endosperm  and  fleshy  cotyle- 
dons of  seeds,  and  in  Irish  Potatoes,  Sweet  Potatoes,  and  in  other 
tubers  and  roots  where  the  cells  enlarge  and  become  packed  with 
food.  In  the  pith  some  water  is  usually  stored  and  often  much 
food,  as  is  well  known  in  the  case  of  Sugar  Cane  and  Sorghum  in 
which  the  pith  contains  much  sugar.  Throughout  the  wood  of 
trees  there  are  thin-walled  living  cells,  forming  the  medullary  rays, 
which  function  as  a  storage  tissue.  In  Maple  trees  the  sugar 
occurring  in  the  spring  sap  comes  from  the  starch  which  was 


REPRODUCTIVE  TISSUES  133 

stored  chiefly  in  the  medullary  rays  during  the  previous  season. 
For  water  storage  some  plants  have  special  tissues,  while  others 
hke  the  Cacti  store  it  throughout  the  plant  body. 

Secretory  Tissues.  —  Secretory  tissues,  although  not  so  essential 
and  no  so  common  among  plants  as  the  other  tissues  discussed, 
perform  an  important  function  in  some  cases.  Most  showy 
flowers  have  secreting  tissues,  known  as  necta  glands,  located  at 
the  base  of  the  corolla  or  calyx.  (Fig.  121.)  These  glands  secrete 
the  nectar,  which,  by  attracting  insects,  aids  in  securing  cross- 
pollination.  Furthermore,  honey  is  made  from  nectar,  and  the 
value  of  a  plant  as  a  bee-plant  depends  upon  the  amount  and  qual- 
ity of  nectar  secreted  by  its  nectar  glands.  On  the  leaves,  stems, 
or  fruits  of  many  plants,  such 
as  Mints,  Oranges,  Lemons,  etc., 
there  are  glands  whose  secre- 
tions give  the  planL  a  peculiar 
fragrance.  In  he  stems  and 
leaves  of  Conifers  occur  long 
tubes  or  ducts,  known  as  resin 
ducts,  which  are  lined  with  secre- 
tory cells  that  secrete  resin  from 
which  pine  tar,  rosin,  turpen- 
tine, and  other  valuable  prod- 
ucts are  made.  Much  hke  the  Fig.  121.  —  A  Buckwheat  flower 
resin  ducts  are  the  milk  or  lac-  with  sepals  removed  from  one  side 
tiferous  vessels  of  the  Milkweeds  ^^  ^1^°]^  ^^e  nectar  glands  (n).  After 
(Asclepiadaceae) ,  Spurges  (Eu- 
phorbia ceae) ,  Dogbanes  Apocynaceae) ,  and  other  plant  families 
where  milk-like  secretions  occur.  There  are  numerous  secretions 
many  of  which,  however,  are  secreted  by  cells  in  which  secreting 
is  not  the  special  fimction. 

Reproductive  Tissues.  —  Reproductive  structures  are  of  two 
kinds,  sexual  and  asexual.  Any  portion  of  a  plant,  as  a  bud, 
tuber,  stem,  or  root  which  may  function  in  producing  new  plants, 
is  regarded  as  an  asexual  reproductive  structure.  Some  plants, 
as  Irish  Potatoes,  Sweet  Potatoes,  and  Strawberries  illustrate, 
are  quite  generally  propagated  asexually. 

In  the  higher  plants  the  sexual  reproductive  tissues  are  those 
of  the  flower,  and  more  especially  those  of  the  stamens  and  pistils 
with  which  the  student  is  familiar.     Although  the  eggs  and  sperms 


134  CELLS  AND   TISSUES 

are  the  chief  reproductive  cells,  all  the  tissues  of  the  stamens  and 
pistils  are  related  to  fertilization,  which  is  the  chief  feature  in 
sexual  reproduction. 

In  plants  like  Ferns,  Mosses,  and  Algae,  where  there  are  no 
flowers,  the  sex  cells  are  commonly  borne  in  special  organs,  called 
sex  organs,  which  are    so  constructed  as  to  favor  fertilization. 

Summary  of  the  Cell  and  Tissues.  —  The  cell  is  the  unit  of 
plant  and  animal  life.  It  contains  the  living  substance,  known 
as  protoplasm,  which  is  usually  enclosed  in  a  cell  wall.  The  pro- 
toplasm is  composed  of  a  nucleus  and  cytoplasm.  Cells  receive 
water,  food,  and  mineral  elements  through  osmosis  and  ordi- 
nary diffusion,  and  obtain  energy  through  respiration.  Cells 
multiply  by  division.  Cell  multiplication  is  accompanied  by 
cell  modifications  which  result  in  the  differentiation  of  the  cells 
into  tissues.  Some  tissues  consist  of  only  the  modified  cell 
walls,  the  protoplasm  having  died  and  disappeared  after  the 
modifications  of  the  walls  are  complete.  The  higher  plants  have 
many  tissues,  each  of  which  has  one  or  more  functions.  The 
meristematic  tissues  enable  the  plant  to  continue  growing;  epi- 
dermal and  cork  tissues  protect  the  plant  from  drying  and  from 
attacks  of  destructive  organisms ;  collenchyma  tissue,  bast  fibers, 
and  wood  fibers  enable  the  plant  to  support  itself  in  a  favor- 
able position  in  spite  of  the  force  of  gravity  and  winds;  vas- 
cular bundles,  hke  the  circulatory  system  of  animals,  supply  the 
other  tissues  with  materials;  the  absorbing  tissues  take  from  the 
soil,  or  other  substrata,  the  water  and  dissolved  substances  which 
the  plant  must  have;  the  storage  tissues  hold  the  water  or  food 
in  reserve  for  future  use.  The  stored  water  is  used  during  dry 
seasons,  and  the  stored  food  is  used  for  the  growth  of  new  plants, 
as  in  case  of  seeds,  tubers,  etc.,  or  for  the  new  growth  of  leaves 
and  flowers  at  the  end  of  a  dormant  period,  as  in  case  of  trees. 
The  food-making  tissues  furnish  the  food  which  all  parts  of  the 
plant  must  have.  Secretory  tissues  assist  in  cross-pollination 
by  providing  secretions  which  attract  insects,  furnish  nectar  from 
which  honey  is  made,  and  give  us  many  other  products,  such  as 
resin,  turpentine,  etc. 


CHAPTER  VIII 

/ 

ROOTS 
General  Features  of  Roots 

The  higher  plants  consist  of  roots  and  shoots.  The  roots  are 
generally  underground  structures,  while  the  shoot  is  the  aerial 
portion  consisting  of  the  stem  with  its  leaves,  buds,  flowers,  and 
fruit.  Plants  like  the  Algae,  which  live  in  the  water  where  all  parts 
of  the  plant  can  absorb  directly  from  the  surroundings,  do  not 
need  roots,  although  they  often  have  structures  known  as  hold- 
fasts which  anchor  them;  but  holdfasts  are  too  simple  in  struc- 
ture to  be  called  roots.  Even  in  the  Mosses,  which  are  mainly 
land  plants,  instead  of  roots  there  are  hair-like  structures,  called 
rhizoids,  which  anchor  the  plant  to  the  substratum.  True  roots 
are  complex  structures  and  are  characteristic  of  Ferns  and  Seed 
Plants. 

Although  we  think  of  roots  as  underground  structures,  there 
are,  however,  a  few  plants  having  roots  adapted  to  living  in  other 
situations,  as  in  the  water,  air,  or  the  tissues  of  other  plants. 
But  with  few  exceptions  our  cultivated  plants  depend  upon  soil 
roots,  which,  therefore,  deserve  most  attention. 

Being  underground  structures,  soil  roots  normally  arise  from 
the  stem's  base,  from  which  they  radiate  by  elongating  and  grow- 
ing new  branches,  which  in  turn  branch  and  rebranch  until  the 
soil  about  the  plant  is  quite  thoroughly  invaded  by  its  root  s.ys- 
tem.  Usually  a  plant's  root  system,  tapering  into  numerous 
branches  almost  hair-like  in  size,  is  more  branched  and  spreads 
farther  horizontally  than  its  stem  system.  The  profuseness 
with  which  roots  branch  is  well  shown  by  the  estimated  root 
length  of  some  plants.  Thus  the  length  of  all  the  roots  of  a  single 
Wheat  or  Oat  plant,  laid  end  to  end,  is  estimated  at  1600  feet,  or 
more  than  a  quarter  of  a  mile.  For  a  vigorous  Corn  plant  the 
estimated  root  length  is  more  than  a  mile.  Certainly  in  some 
trees  the  root  length  would  much  exceed  that  of  Corn. 

The  size  of  a  plant's  root  system,  in  general,  varies  with  that  of 
135 


136  ROOTS 

the  shoot,  for  the  larger  the  shoot,  the  larger  the  root  system  nec- 
essary to  supply  the  adequate  amount  of  water  and  mineral  mat- 
ter, and  to  furnish  sufficient  anchorage.  As  to  the  size  of  the 
roots  of  a  plant,  that  depends  upon  the  size  of  the  shoot,  the  num- 
ber of  roots,  and  the  distances  of  roots  from  the  stem.  The  re- 
lation of  the  size  of  roots  to  that  of  the  shoot  is  well  shown  in  case 
of  trees,  where  the  roots  directly  connected  with  the  stem  and 
known  as  mai7i  roots  increase  in  diameter  from  a  few  millimeters 
often  to  a  foot  or  more  as4rhe  shoot  passes  from  the  seedling  to  the 
mature  stage.  Where  there  is  only  one  main  root,  as  in  Alfalfa 
and  the  Dandelion,  its  size  is  directly  in  proportion  to  the  size  of 
the  shoot,  usually  being  as  large  or  even  larger  in  diameter  than 
the  short  stem  of  the  crown.  On  the  other  hand,  when  the  roots 
leading  from  the  stem  are  numerous,  as  in  tjie  Grasses  and 
numerous  other  plants,  all  are  relatively  small.  As  to  the  size 
of  a  branch  root,  that  depends  much  upon  its  distance  from  the 
stem  or  main  root,  for  all  roots  branch  and  rebranch  until  the 
branches  are  fibrous-like,  usually  being  a  millimeter  or  less  in 
diameter  at  their  tips.  It  is  in  connection  with  these  fiber-hke 
branches,  which  are  the  absorptive  regions,  that  roots  show  most 
uniformit}^;  for  the  roots  of  all  plants  taper  down  to  these  fiber- 
like branches,  which  are  practically  uniform  in  size  for  all  plants. 
This  unifoi'mity  in  size  is  probably  due  to  the  fact  that  only 
roots  with  a  very  small  diameter  are  efficient  absorbers. 

The  texture  of  roots  is  always  soft  at  the  tips  where  the  cells 
active  in  division,  elongation,  and  absorption  have  thin  cellulose 
walls,  which  readily  yield  to  pressure  or  strains.  But  not  far  back 
of  the  absorptive  region  there  are  formed  strengthening  fibers, 
which  afford  a  toughness  that  enables  the  root  to  endure  the 
strains  in  connection  with  its  anchorage  function.  Furthermore, 
roots,  in  their  older  regions,  are  covered  with  cork  which  adds 
firmness  to  the  texture.  In  shrubs  and  trees  the  roots,  in  their 
older  regions,  become  as  woody  and  just  as  hard  as  the  stems. 

As  to  duration,  roots  may  be  short-lived,  serving  the  plant  only 
in  the  seedling  stage,  as  in  case  of  temporary  roots,  or  they  may 
last  as  long  as  the  plant,  as  in  case  of  permanent  roots.  The  life 
of  permanent  roots  is  one,  two,  or  many  years  according  to 
whether  or  not  the  plant  is  annual,  biennial,  or  perennial. 

Interdependence  of  Shoot  and  Root.  —  Upon  the  roots  the 
shoot  depends  for  water,  mineral  matter,  and  anchorage,  while 


INTERDEPENDENCE  OF  SHOOT  AND  ROOT     137 

upon  the  shoot  the  root  system  dejxnids  for  food.  Neither 
could  survive  without  the  other.  Moreover,  if  either  is  hindered 
in  its  development,  the  other  hkewise  will  be  stunted.  For  this 
reason  when  pots  in  case  of  potted  plants  prevent  the  further 
development  of  the  root  system,  the  growth  of  the  shoot  is  checked 
and  the  plant  has  to  be  repotted.  Again,  the  cutting  away  of  the 
roots  of  a  shade  tree  in  excavating  for  a  sewer  or  sidewalk  often 
kills  the  tree  due  to  diminished  water  supply.  A  number  of  in- 
stances can  be  cited  to  show  the  dependence  of  the  roots  upon  the 
shoot.  For  example,  it  is  well  known  that  the  roots  of  Asparagus 
will  not  make  a  good  growth  unless  the  shoots  are  allowed  to 
grow  during  a  part  of  the  summer,  in  order  that  food  may  be  pro- 
vided for  the  growth  of  the  roots.  Furthermore,  it  is  a  common 
practice  in  eradicating  such  weeds  as  Canada  Thistle  and  Quack 
Grass,  to  starve  the  underground  structures  by  keeping  down  the 
shoots.  To  enable  plants  to  establish  a  good  root  system,  in 
order  that  there  may  be  a  well  developed  shoot,  is  one  of  the 
chief  aims  in  cultivation. 

The  most  necessary  material  absorbed  by  plants  is  water,  which 
is  supplied  almost  entirely  by  the  roots  in  higher  plants,  and 
serves  at  least  a  half  dozen  different  purposes.  First,  water  is 
necessary,  and  in  large  quantities  too,  to  prevent  the  shoot  from 
becoming  dried  out  through  loss  of  water  to  the  surrounding  air. 
Leaves  and  also  stems,  unless  the  latter  are  well  covered  with 
bark,  are  constantly  having  water  evaporated  from  them  and, 
unless  this  loss  is  compensated,  the  shoot  will  soon  die.  Second, 
water  enables  the  cells  to  maintain  their  turgidity,  which  main- 
tains the  leaves  and  other  soft  tissues  of  the  shoot  in  a  rigid  posi- 
tion, and  thus  in  a  position  suitable  for  work.  Third,  water  is  an 
essential  constituent  of  sugar,  starch,  and  other  foods  made  by  the 
shoot.  Fourth,  water  as  the  plant's  solvent  is  the  medium  through 
which  substances  in  solution  are  distributed  through  the  plant. 
Thus  through  water  as  a  medium,  the  mineral  elements  of  the  soil 
and  the  foods  made  in  the  leaves  are  carried  to  all  parts  of  the 
plant.  Fifth,  it  is  in  the  form  of  a  solution  in  water  that  sub- 
stances in  the  plant  react  chemically  with  each  other.  Sixth, 
water  is  an  important  constituent  of  protoplasm,  cell  walls,  and 
other  plant  structures,  usually  being  more  than  90  per  <;ent  of 
their  fresh  weight.  Thus  it  is  no  wonder  that  plants  must  have 
water  or  they  soon  perish. 


138 


ROOTS 


In  anchor ing  the  shoot  most  soil  roots  perforin  an  important 
function,  except  in  those  plants  with  stems  prostrate  on  the  ground 
or  climbing  supports.  In  plants  with  upright  stems,  as  in  trees, 
the  strains  due  to  winds  and  gravity  when  the  plant  is  bearing 
foliage  and  fruit  is  often  enormous.  However,  the  root  system 
is  usually  able  to  hold  the  plant  in  place,  although  the  strains 
may  break  off  })ranches  or  even  the  main  stem.  It  is  by  spread- 
ing laterally  and  profusely  branching,  that  roots  become  so  firmly 
attached  to  large  masses  of  soil  that  they  can  endure  enormous 
strains. 

In  addition  to  anchoring  the  plant  and  furnishing  it  water  and 
mineral  matter,  in  many  plants  the  roots  function  as  storage  or- 
gans, in  which  some  of  the  food  made  by  the  shoot  each  year  is 
stored  for  use  in  the  development 
of  new  shoots  each  succeeding  year. 
This  function  is  especially  obvious 
in  many  plants  which  die  down  in 
the  fall  and  grow  up  again  in  the 
spring. 

Thus  the  root  depends  upon  the 
shoot  for  food  while  the  shoot  de- 
pends upon  the  root:  (1)  for  water 
and  mineral  matter;  (2)  for  an- 
chorage; and  (3)  often  as  a  storage 
organ. 

Types  of  Root  Systems.  —  There 
are  various  irregularities  among 
root  systems,  due  to  the  altera- 
tions which  a  root  system  must 
make  in  adjusting  itself  to  obstruc- 
tions and  the  uneven  distribution 
of  water  and  mineral  matter  in  the 
soil.  For  this  reason  root  systems 
are  less  symmetrical  than  shoots. 
However,  despite  these  irregulari- 
ties there  are  some  inherent  differ- 
ences that  are  so  regular  as  to  be  typical  of  certain  plants. 

In  the  Corn,  Wheat,  Oats,  and  Grasses  in  general,  there  is  the 
type  of  root  system,  known  as  the  fibrous  root  system,  in  which 
there  are  no  dominant  main  roots,  but  all  roots  are  small  and  with 


Fig.  122.  —The  fibrous  roots 
of  Corn. 


TYPES  OF   ROOT  SYSTEMS 


139 


their  numerous  fine  branches  form  a  system  resembhng  a  fine 
brush  when  the  dirt  is  washed  away.  (Fig.  122.)  This  type  of 
root  system  is  common  among  weeds,  trees,  and  .  w>. 
many  cultivated  plants.  In  addition  to  the  un- 
derground roots,  some  of  the  Grasses,  as  Corn 
illustrates,  develop  jprop  or  brace  roots,  which 
grow  out  from  nodes  above  the  ground  into 
which  they  finally  reach  to  afford  additional  an- 
chorage. However,  brace  roots  are  not  neces- 
sarily an  accompaniment 
of  fibrous  root  systems, 
for  they  may  occur  in 
connection  with  other 
kinds  of  root  systems. 
The  tap-root  system,,  in 
^^-  /  i*ip  which  there  is  one  large 
^^-^^    ^^  main    root    from    which 

small  lateral  branches 
arise,  is  typical  of  the 
Alfalfa,  Red  Clover, 
Beets,  Dandelion,  and 
numerous  other  plants. 
Tap-roots  usually  grow 
directly  downward,  pen- 
etrating into  the  deeper 
layers  of  the  soil  where 
more  moisture  is  avail- 
able. (Fig.  123.)  For 
this  reason,  the  tap-root 
system  is  best  adapted  for  dry  regions 
and  is,  therefore,  characteristic  of  drought 
resistant  plants.  Although  the  tap-root 
is  more  common  among  herbaceous 
plants,  it  occurs,  nevertheless,  among 
trees,  where  it  often  interferes  with  trans- 
planting, as  in  case  of  Hickories,  Oaks, 
and  Maples.     (Fig.  124.) 

Tap-roots  are  also  convenient  storage 
organs  in  which  food  is  stored  for  the  growth  of  the  new  shoot 
the  next  year.     This  fact  is  well  illustrated  in  Alfalfa,  Clover, 


Fig.  123.  — 
AKalfa,  a  plant 
with  a  promi- 
nent tap-root. 


Fig.  124.— Young  Shell- 
bark  Hickory,  showing  the 
tap-root.  After  Farmers' 
Bulletin  173,  U.  S.  Dept. 
of  Agriculture. 


140 


ROOTS 


Fig.  125.  —  Sugar  Beet,  a 
plant  with  a  fleshy  tap-root. 


and  the  Dandelion,  where  the  shoots  die  down  in  the  fall  to  be 
followed  b}'  new  ones  in  the  spring.  Thus  the  tap-root  sj^stem 
is  well  adapted  to  the  perennial  habit.  In  some  plants,  as 
Radishes,  Beets,  Carrots,  Turnips, 
etc.,  where  the  storage  function  is 
quite  prominent,  the  tap-root  is 
tender  and  of  much  importance  as 
a  vegetable.  {Fig.  125.)  From  some 
fleshy  roots  valuable  products  are 
extracted,  notably  the  Sugar  Beet 
from  which  most  of  our  sugar  is  ob- 
tained. 

Plants  having  prominent  tap-roots 

with  short  lateral  roots  can  be  grown 

close  together  without  injury.     Due 

to  this  fact  and  to  the  size  of  the 

shoot,  such  plants  as  Clover,  Alfalfa, 

Beets,  and  others  with  the  tap-root 

system  grow  well  when  crowded. 

The  fascicled  root-system,  consisting  of  a  cluster  of  roots  all  of 

which  are  much  enlarged  in  connection  with  the  storage  of  food, 

is  characteristic  of  a  few  plants  of  which  the  Sweet  Potato  and 

Dahlia  are  two  that  are  well  known.     {Fig.  126.) 

Adventitious  roots,  so  named  because  of  their  occurrence  in  un- 
accustomed places,  may  be 
mentioned  here,  although 
the  classification  pertains 
to  the  place  of  occurrence 
and  not  to  any  peculiar  fea- 
ture of  the  root  itself;  for 
any  root,  whether  fleshy  or 
fibrous,  developing  from 
leaves  or  from  stem  regions 
where  roots  are  not  nor- 
mally present  is  called  ad- 
ventitious. All  roots  may 
be  regarded  as  adventitious 
except  those,  known  as  the  primary  ones,  which  develop  directly 
from  the  radicle  of  the  embryo. 

The  ability  of  many  shoots  to  develop  roots  from  various  re- 


FiG.  126.  —  A  portion  of  a  Sweet  Potato 
plant,  showing  the  fascicled  roots. 


DEPTH  AND  SPREAD  OF  ROOT  SYSTEMS      141 

gions  of  the  stem  is  of  much  service  in  plant  propagation.  When 
canes  of  some  varieties  of  Raspberries  bend  over  and  touch  the 
ground,  they  become  rooted  at  their  tips.  If  the  canes  are  cut,  the 
rooted  tip  then  becomes  a  new  plant.  This  is  a  common  method 
of  propagating  Raspberries.  If  the  branches  of  the  Grape  Vine, 
or,  if  many  of  our  shrubs  are  bent  to  the  ground  and  a  portion 
covered  with  soil,  roots  will  develop  on  the  buried  portion,  which 
thereby  becomes  a  means  of  obtaining  new  plants.  Geraniums, 
Coleus,  Roses,  and  many  other  plants  are  propagated  by  cutting 
off  branches  and  setting  them  in  moist  sand  where  they  develop 
adventitious  roots  and  l3ecome  new  plants. 

Depth  and  Spread  of  Root  Systems.^  —  Roots  must  go  deep 
enough  and  spread  far  enough  laterally  to  meet  the  demands  of 
the  plant  for  absorption  and  anchorage,  both  of  which  in  general 
must  conform  to  the  size  of  the  shoot.  On  this  account,  trees 
need  a  deeper  and  wider  root  system  than  a  Corn  plant.  But 
aside  from  these  differences  which  relate  to  the  size  of  the  shoot, 
root  systems  of  different  plants  differ  in  the  depth  and  spread 
according  to :  (1)  the  conditions  of  the  soil  in  relation  to  moisture, 
mineral  matter,  and  air;  (2)  the  type  of  root  system;  and  (3)  the 
difference  in  the  disposition  of  the  roots  of  different  plants,  al- 
though similar  in  type. 

Roots,  like  all  other  plant  portions  containing  living  cells, 
must  have  oxygen  for  respiration.  For  this  reason  the  region  of 
the  soil  just  under  the  surface  where  air  is  accessible  is  more  fav- 
orable for  root  activity  than  the  deeper  soil  regions.  Besides, 
more  of  the  necessary  mineral  matter  is  available  in  the  surface 
layers  of  the  soil.  Consequently,  root  systems  increase  by  ex- 
tending proportionately  much  more  laterally  than  downward, 
except  in  cases  where  there  is  extensive  development  of  a  tap- 
root, as  in  such  plants  as  Alfalfa  and  the  Mesquite. 

Studies  made  of  the  roots  of  Corn  show  that  under  ordinary 
conditions  the  roots  extend  laterally,  most  of  them  being  only 
from  3  to  6  inches  under  the  surface,  until  they  reach  a  distance 
of  about  1|  feet  from  the  plant,  and  then  they  extend  downward 
as  well  as  laterally,  often  having  a  depth  of  3  or  4  feet  when  the 

1  The  Roots  of  Plants.  Bulletin  127,  Kansas  Agr.  Exp.  Sta.  Root  Sys- 
tems of  Field  Crops.  Bulletin  64,  N.  Dakota  Agr.  Exp.  Sta.  Corn,  its  Habit 
of  Root  Growth,  Methods  of  Planting  and  Cultivating,  Notes  on  Ears  and 
Stools  or  Suckers.     Bulletin  5,  Minnesota  Agr.  Exp.  Sta. 


142  ROOTS 

plant  is  mature.  In  the  Irish  Potato  the  roots  may  reach  a  depth 
of  3  feet  after  extending  laterally  2  or  3  feet,  but  for  most  of  their 
length  they  are  within  a  few  inches  of  the  surface.  It  may  now 
be  seen  that  deep  cultivation  may  injure  Corn,  Potatoes,  and 
other  plants  with  a  similar  root  habit  by  tearing  away  the  roots. 
In  Wheat,  Oats,  and  Barley,  the  root  systems  do  not  extend  so 
far  laterally  as  in  Corn  but  deeper  into  the  soil,  reaching  a  depth  of 
4  to  5  feet.  Flax  and  Kafir  Corn  have  fibrous  root  systems  which 
feed  mainly  from  the  surface  soil.  Plants  with  shallow  roots  are 
called  surface  feeders,  and  are  considered  "  hard  on  the  land,"  be- 
cause they  exhaust  the  moisture  and  mineral  matter  in  the  sur- 
face soil. 

In  trees,  although  the  lateral  roots  may  reach  a  length  of  nearly 
100  feet,  they  still  remain  near  the  surface.  In  the  Soft  Maple, 
lateral  roots  80  feet  in  length  have  been  found  to  range  in  depth 
from  8  inches  to  2  feet.  In  old  Apple  trees  the  lateral  roots, 
which  may  be  60  feet  of  more  in  length,  usually  have  a  depth 
ranging  from  2  to  5  feet.  Since  trees  by  the  surface  habit  of  their 
roots  take  the  moisture  and  mineral  matter  from  near  the  surface, 
it  is  clear  why  crops  do  not  grow  well  around  them  even  when 
not  affected  by  their  shade. 

Some  fruit  trees,  such  as  the  Cherry  and  Pear,  send  their  roots 
several  feet  into  the  soil  and,  therefore,  require  a  deeper  soil  than 
some  other  kinds  of  fruit  trees.  The  Quince,  commonly  used  as 
a  stock  on  which  to  graft  Pears,  has  a  shallow  root  system,  and  so 
has  the  ' '  Paradise ' '  Apple  on  which  Apples  are  often  grafted.  The 
fact  that  Pears  grafted  on  Quinces  or  Common  Apples  grafted  on 
the  "Paradise/'  Apple  bear  younger  than  they  do  when  grown  on 
their  own  roots,  shows  that  the  shoot  and  root  system  are  very 
closely  related  in  their  activities.  The  deep  root  systems  occur 
generally  in  connection  with  tap-roots,  which  sometimes  reach 
extraordinary  depths.  Also  the  lateral  roots  in  a  tap-root  system 
are  usually  well  under  the  surface.  For  example,  in  Sugar  Beets 
the  lateral  roots  are  6  inches  or  more  under  the  surface,  and, 
therefore,  not  usually  disturbed  by  deep  cultivation.  On  account 
of  having  a  deep  root  system,  Beets  require  a  deep  and  well 
loosened  soil. 

As  to  the  depth  reached  by  tap-roots,  5  or  6  feet  is  common  in 
Alfalfa,  and  a  depth  of  31  feet  has  been  recorded.  The  tap-root 
of  the  Mesquite,  which  is  a  native  of  desert  regions,  has  been 


CELLULAR  ANATOMY  OF   THE   ROOT   TIP 


143 


known  to  reach  a  depth  ot"  (30  feet.  Plants  with  tlie  Alfalfa  type 
of  root  system  are  not  only  drought  resistant,  but  also  loosen  the 
subsoil,  which  is  thereby  put  in  better  condition  for  those  plants 
with  roots  less  able  to  penetrate  a  hard  subsoil. 


Root  Structure  ^ 

As  the  student  already  knows,  plant  organs  consist  of  tissues, 
each  of  which  on  account  of  the  peculiar  structure  of  its  cells  is 
especially  adapted  to  do  a  certain  kind  of  work. 
In  roots  there  are  tissues  to  perform  the  following 
functions:  (1)  protection;  (2)  growth;  (3)  absorp- 
tion; (4)  conduction;  and  (5)  strengthening.  Root 
tips  show  on  their  surface  rather  distinct  regions, 
which  differ  in  color,  texture,  or  some  other  feature 
that  can  be  seen  without  a  microscope.  Often,  but 
not  always,  the  small  protective  cap,  which  is  the 
actual  end  of  the  root,  can  be  identified  by  its 
brownish  color.  The  smooth  whitish  zone,  which  is 
usually  a  conspicuous  region  of  the  tip,  is  where 
cell  multiplication  and  growth  are  most  prominent. 
Just  back  of  this  is  the  absorptive  zone,  bearing 
numerous  root  hairs  which  are  more  conspicuous 
when  grown  in  moist  air  or  moss  where  there  are  no 
soil  particles  to  influence  their  shape.  {Fig.  127.) 
Back  of  the  absorptive  region,  where  protective  and 
strengthening  tissues  are  becoming  prominent,  the 
root  is  firmer  in  texture  and  darker  in  color;  and 
these  features  become  more  prominent  with  age,  as  is  well  dem- 
onstrated in  shrubs  and  trees  where  the  older  parts  of  roots  are 
w^oody  and  covered  with  thick  bark. 

Cellular  Anatomy  of  the  Root  Tip.  —  If  with  the  aid  of  a  micro- 
scope a  lengthwise  section  through  a  root  tip  is  studied,  more  may 
be  learned  about  the  character  of  the  different  tissues.  {Fig.  128.) 
The  root  cap  now  appears  as  a  well  defined  structure,  consisting 
of  many  cells  loosely  joined  into  a  covering,  which  is  thickest 
directly  over  the  end  of  the  root.  Next  to  the  root  cap  is  the 
zone  of  cells  active  in  division  and  constituting  meristematic  tissue. 
Back  of  this  is  the  growth  zone,  in  which  the  chief  activitj^  is  cell 
enlargement  to  which  the  elongation  of  the  root  is  due.     Other 


144 


ROOTS 


'H 


^^'It-i^y     ^^V  cJ 


Fig.  128.  —  Longitudinal  sections  through  rcot  of  Onion  at  the  following 
regions.  A,  through  the  tip  showing  the  root  cap  and  meristematic  zone. 
B,  through  the  zone  of  elongation.  C,  through  absorptive  region  or  hair  zone. 
Highly  magnified. 


CELLULAR  ANATOMY  OF   THE   ROOT   TIP 


145 


regions  of  the  root  increase  in  diameter,  but  almost  all  elongation 
takes  place  in  the  growth  zone,  as  shown  in  Figure  129.  The 
meristematic  zone  is  thus  so  situated  that  the  new  cells  formed 
by  it  may  be  added  both  to  the  root  cap,  the  thickness  of  which 
is  thereby  maintained  in  spite  of  its  being  rapidly  worn  away  on 
its  outer  surface,  and  to  the  growth  zouv^,  the  older  portions  of 
which  are  constantly  taking  on  the  fea- 
tures of  the  absorptive  zone  just  behind. 
The  growth  zone  merges  imperceptibly 
into  the  absorptive  zone  where  the  fol- 
lowing tissues  become  quite  well  defined: 
(1)  a  surface  layer  of  cells  constituting 
the  epidermis  which  has  most  to  do  with 
absorption,  the  special  absorptive  agents 
being  the  root  hairs,  which,  as  the  section 
shows,  are  merely  projections  of  the  epi- 
dermal cells;  (2)  a  broad  band  of  cells 
just  beneath  the  epidermis  and  constitut- 
ing the  cortex;  and  (3)  a  group  of  con- 
ductive tissues  forming  a  central  cylinder, 
known  as  the  vascular  cylinder. 

It  is  to  be  noted  that  the  epidermis  of 
roots,  unlike  that  of  leaves  and  stems, 
has  no  cutinized  walls  and  contains  no 
stomata  or  other  openings  for  the  entrance 
of  air,  although  so  many  active  cells  re- 
quire much  oxygen  for  respiration.  How- 
ever, openings  are  not  necessary,  for  the 
uncutinized  walls  offer  practically  no  re- 
sistance to  the  passage  of  water,  which 
usually  carries  in  solution  oxygen  enough 
to  support  quite  active  respiration. 

Through  the  development  of  the  root  hairs  the  absorptive 
surface  of  the  root  system  is  much  increased,  and  may  be  thereby 
increased  from  five  to  six  times  in  Corn,  about  twelve  times  in 
Barley  and  as  much  as  eighteen  times  in  some  other  plants.  All 
root  hairs  are  able  to  absorb  regardless  of  their  size,  which  ranges 
from  a  slight  bulge  near  the  growth  zone  of  the  root  to  often 
more  than  an  inch  farther  back.  They  live  only  a  few  days, 
but,  as  they  die  off  behind,  new  ones  form  ahead,  and  in  this 


Fig.  129.  —  The  radi- 
cle of  a  Corn  seedling 
marked  to  show  the  re- 
gions of  elongation.  .4, 
radicle  just  after  being 
divided  into  spaces  of 
about  -/o  of  ai^  inch  in 
width.  B,  radicle  sev- 
eral hours  after  mark- 
ing, showing  the  region 
where  elongation  is  tak- 
ing place.  Modified  from 
Andrews. 


146  ROOTS 

way  the  absorptive  zone  moves  along  just  behind  the  advancing 
tip.  The  root  hairs  grow  out  more  or  less  at  right  angles  to  the 
surface  of  the  root,  and  are  able  on  account  of  their  flexible 
slimy  walls  to  push  through  small  openings  and  around  the  soil 
particles  against  which  they  flatten  and  to  which  they  become 
glued  fast,  thereby  coming  in  close  contact  with  the  water  films 
around  the  soil  particles.  Why  the  region  of  elongation  is  near 
the  tip  is  now  clear,  for,  if  behind  the  absorptive  zone,  the  hair 
region  would  be  pushed  ahead  and  the  root  hairs  thereby  torn 
away. 


Fig.  130.  —  Cross  section  of  a  root  through  the  absorbing  region,  x, 
xylem;  p,  phloem;  a,  pericycle;  e,  endodermis;  c,  cortex;  h,  root  hairs. 
Highly  magnified. 

The  cortex,  consisting  of  many  layers  of  parenchyma  cells, 
transports  the  substances  absorbed  by  the  root  hairs  to  the  con- 
ductive tissues,  and  in  fleshy  roots  also  serves  as  a  storage  region. 

The  vascular  cylinder  contains  the  conductive  tissues,  notably 
the  xylem  and  phloem.  The  xylem  and  phloem  and  their  posi- 
tions in  reference  to  each  other  are  best  seen  in  a  cross  section  of 
a  root,  as  shown  in  Figure  130.  The  xylem  occupies  the  cen- 
ter and  has  strands  radiating  from  the  center  like  the  spokes  of 
a  wheel.  Between  the  spokes  of  the  xylem  and  near  their  outer 
ends  are  the  phloem  strands.  Inasmuch  as  the  absorbed  sub- 
stances are  carried  to  the  shoot  by  the  xylem,  this  alternate  ar- 


ANATOMY  OF  THE  OLDER  PORTIONS  OF  THE  ROOT   147 


4-.A;^ 


rangement  of  xyleni  and  pliloem  shows  adaptation,  in  that  it  per- 
mits the  absorbed  substances  to  reach  the  xylem  without  passing 
through  the  phloem.  The  vascular  cylinder  is  bordered  by  a 
chain  of  cells,  known  as  the  pericycie.  The  pericycle  joins  the 
endodermis  or  starch  sheath  which  is  the  chain  of  cells  forming 
the  innermost  layer  of  the  cortex.  Aside  from  the  fact  that 
branches  or  lateral  roots  develop  from  the  pericycle,  the  functions 
of  the  endodermis  and  pericycle  in  roots  are  not  well  understood. 

Anatomy  of  the  Older  Portions  of  the  Root.  —  Not  far  back  of 
the  hair  zone,  as  indicated  by  the  brownish  color  and  the  slough- 
ing off  of  the  epidermis  with  its  dead  root  hairs,  there  appear 
some  anatomical  changes,  such  as  the  formation  of  a  corky  cover- 
ing, enlargement  of  conductive  and  strengthening  tissues,  and 
the  development  of  branches.  As  this  region  of  the  root  becomes 
older,  these  anatomical  changes  become  more  prominent. 

Since  the  epidermis  behind  the  hair  zone  dies  and  falls  away, 
absorption  is  limited  to  the  tip  region  of  the  root.  Accompany- 
ing the  death  of  the  epidermis,  a 
protective  tissue  is  developed  by  the 
layers  of  cells  beneath.  Usually  the 
cells  just  beneath  become  cutinized 
and  take  the  place  of  the  epidermis 
as  a  covering.  In  Grass  roots  the 
layers  of  cells  just  beneath  the  epi- 
dermis thicken  their  walls,  thereby 
forming  over  the  root  a  hard  woody 
rind  similar  to  that  of  Grass  stems. 
Commonly  in  roots  there  is  also 
formed  in  the  region  of  the  pericycle 
a  meristematic  band  of  cells,  known 
as  the  cork  camhmm,  which  by  divid- 
ing parallel  to  the  surface  of  the  root 
adds  layers  of  cork  on  its  outer  side 
and  cortex  cells  on  its  inner  side,  thus 
forming  a  protective  covering  and 
a  secondary  cortex  which  it  also  en- 
larges as  the  root  grows  older.  Since  cork  is  impervious  to  water 
and  the  foods  contained,  by  the  formation  of  cork  in  the  region 
of  the  pericycle,  the  first  or  primary  cortex  has  its  conductive 
connections  with  the  vascular  bundles   cut  off   and  its   death 


Fig.  131.  —  Diagram  of  a 
lengthwise  section  through  the 
region  of  the  root  back  of  the 
hair  zone,  showing  the  changes 
in  the  epidermis  and  cortex,  e, 
epidermis  dead  and  sloughing 
off;  k,  cork  cambium  on  the  in- 
ner side  with  cork  and  dead  cor- 
tex between  it  and  the  epidermis; 
c,  secondary  cortex;  v,  vascular 
cylinder.     Highly  magnified. 


148  ROOTS 

must  follow.  {Fig.  131.)  The  dead  epidermis  and  cortex  form 
the  outer  portion  of  the  bark,  which  thickens  as  cork  is  added  by 
the  cork  cam])ium,  and  in  roots  living  a  number  of  years,  like 
those  of  shrubs  and  trees,  may  become  quite  thick,  and  broken 
and  furrowed,  as  in  the  large  roots  of  trees.  As  a  provision  for 
strength,  fiber-like  strands  of  strengthening  tissue  are  commonly 
formed  in  the  secondary  cortex. 

In  order  that  the  vascular  cylinder  may  have  adequate  con- 
ductive capacity  in  the  older  portions  of  the  root,  it,  too,  must 
enlarge,  for  as  the  absorptive  surface  of  the  root  increases  ahead 
by  the  multiplication  of  branches,  not  only  is  there  an  increase 
in  the  amount  of  absorbed  substances  which  the  xylem  must 
carry  to  the  shoot,  but  also  an  increase  in  the  amount  of  food 
which  the  phloem  must  carry  to  feed  the  greater  number  of 
branches  of  the  root.  In  the  formation  of  xylem  in  roots,  the 
portions  first  formed  are  the  radiating  strands  or  spokes  which 
enlarge  by  developing  toward  the  center  where  they  usually  come 
together,  thus  forming  the  solid  central  core  of  xylem  as  shown 
in  Figure  132.  However,  in  some  plants,  as  in  Corn  and  many 
other  Monocotyledons,  the  xylem  strands  never  come  together, 
and  consequently  a  central  pith  is  left,  around  which  the  strands 
of  xylem  are  arranged.  In  all  roots  the  xylem  strands  are  at  first 
enlarged  by  this  centripetal  development.  In  some  short-hved 
roots  of  Dicotyledons  and  in  the  roots  of  most  Monocotyledons, 
the  enlargement  of  the  xylem  is  due  to  this  centripetal  develop- 
ment and  the  development  of  new  vascular  bundles  between  the 
old  ones  as  the  root  becomes  older.  In  Dicotyledons  and  also 
in  the  Gjrninospenns,  the  group  to  which  Pines,  Firs,  Spruces,  etc., 
belong,  the  vascular  cylinders  of  most  roots  are  increased  also 
as  shown  in  Figure  132.  It  is  seen  in  A  of  Figure  132  that  the 
phloem  and  xylem  are  not  in  contact.  Lying  between  them 
are  cells  which  have  not  been  modified  into  definite  tissues. 
Some  of  these  cells  become  meristematic  and  so  arranged  as  to 
form  a  continuous  band  of  cambium,  known  as  the  cambium 
ring,  which  by  curving  outward  passes  on  the  outside  of  the 
xylem,  and  by  curving  inward  passes  on  the  inside  of  the  phloem, 
thus  separating  the  xylem  and  phloem  regions  of  the  vascular 
cylinder  as  shown  at  B.  The  cambium  cells,  in  the  main,  divide 
parallel  to  the  surface  of  the  root,  and  divide  in  such  a  way  that 
the  layers  of  new  cells  on  the  inside  of  the  cambium  are  about 


ANATOMY  OF  THE  OLDER  PORTIONS  OF  THE  ROOT   149 

equal  in  number  to  those  on  the  outside.  The  new  cells  next  to 
the  xylem  become  modified  into  xylem,  at  first  filling  in  between 
the  strands  of  the  primary  xylem,  and  the  cells  formed  next  to 
the  phloem  are  changed  into  phloem.    In  this  way  the  vascular 


Fig.  132.  —  Diagrams  showing  how  the  xylem  and  phloem  of  roots  are 
increased.  A,  cross  section  of  a  root  showing  xylem  (x)  and  phloem  (p) 
before  the  cambium  is  formed.  B,  cross  section  of  root  showing  xylem  (x) 
and  phloem  (p)  after  the  cambium  ring  (c)  is  formed.  C,  cross  section  show- 
ing the  xylem  (.r)  and  phloem  (p)  shown  in  A  and  B  and  the  new  xylem  (a) 
and  new  phloem  (6)  wliich  have  been  formed  by  the  cambium  (c).  Adapted 
from  J.  M.  Coulter. 


cylinder  is  increased  in  diameter  as  shown  at  C.  In  roots  which 
live  many  years,  like  those  of  trees,  the  layers  of  xylem  formed 
each  year  form  annual  rings  like  those  occurring  in  woody  stems; 
and  the  outer  layers  of  the  phloem,  with  the  cortex  and  other 
tissues  outside  of  the  phloem,  constitute  a  bark  like  that  of 
woody  stems.  In  fact,  it  is  only  by  the  presence  of  their  early 
root  anatomy  that  sections  of  such  roots  can  be  told  from  sec- 
tions of  stems.  In  some  fleshy  roots,  as  Beets  illustrate,  a  num- 
ber of  cambium  rings  form  outside  of  the  first  one,  and  the 
growth  resulting  from  each  appears  as  a  ring  when  a  cross-section, 
of  the  root  is  made. 


150 


ROOTS 


Another  anatomical  feature  connected  with  the  older  portions 
of  roots  is  the  development  of  branches,  which  begin  to  develop 
some  distance  back  of  the  hair  zone  and 
in  the  way  shown  in  Figure  133.  In  Seed 
Plants  the  branch  roots,  which  are  called 
secondary,  tertiary,  and  so  on  according  to 
their  distance  from  the  main  root,  develop 
from  the  pericycle  and  usually  in  the  re- 
gion closest  to  the  xylem.  In  forming  a 
new  branch,  a  few  cells  of  the  pericycle  in 
the  region  where  the  branch  is  to  appear 
begin  to  divide  parallel  to  the  surface  of 
the  root.  The  new  cells  at  first  appear  as 
a  slight  elevation  on  the  pericycle,  but  by 
rapid  growth  this  elevation  of  cells  soon 
pushes  through  the  cortex  and  other  over- 
lying tissues,  and  becomes  a  branch  with 
vascular  cylinder  and  other  tissues  con- 
tinuous with  those  of  the  root  of  which  it 
is  a  branch.  Of  course  the  farther  from 
the  root  tip,  the  older  and  more  fully  de- 
veloped are  the  branches. 

One  important  feature  in  connection  with 
the  branching  habit  is  that,  when  the  end 
of  a  root  is  cut  away,  the  remaining  por- 
tion is  stimulated  to  develop  branches.  It 
is  due  to  the  ability  of  roots  to  branch, 
that  trees  and  other  plants  with  their  roots 
heavily  pruned  in  transplanting  are  usually 
able  to  provide  a  new  root  system  and  become  established  in 
their  new  location. 


Fig.  133.  — LengtH^ 
wise  section  through 
a  root,  showing  how 
branches  arise.  The 
branches  (6)  originate 
in  the  region  of  the 
vascular  cyhndcr  and 
push  through  the  cor- 
tex, finally  reaching 
the  exterior. 


Factors  Influencing  the  Direction  of  Growth  in  Roots 

Roots  and  stems  respond  very  differently  in  respect  to  gravity. 
Primary  roots  grow  toward  the  center  of  gravity,  while  most 
stems  grow  in  the  opposite  direction.  This  earth  influence  is 
known  as  geotropis?n.  Geo  comes  from  a  word  meaning  earth 
and  tropism  means  turning.  So  the  word,  geotropism,  means 
earth-turning,  and  refers  to  the  turning  of  the  root  and  stem  in 


FACTORS  INFLUENCING   THE   DIRECTION  OF  GROWTH       151 

response  to  the  influence  of  gravity.  Primary  roots  are  positively 
geotropie  (growing  toward  the  earth's  center),  while  most  stems 
are  negatively  geotropie  (growing  directly  away  from  the  earth). 
Lateral  roots,  as  well  as  most  bi-anches  of  stems,  grow  more  or  less 


Fig.  134.  —  The  radicle  or  primary  root  of  the  Sunflower  growing  downward 
in  response  to  gravity.     After  Osterhout. 

horizontall}^  and,  when  strictl}^  horizontal,  are  neither  positively 
nor  negatively  geotropie.     What  is  shown  in  Figure  134? 

Roots,  especially  primary  roots,  are  sensitive  to  moisture  and 
grow  towards  it  when  more  moisture  is  needed.  The  tropism 
induced  by  water  is  called  hydrotropism.  Most  roots  are  to  a 
greater  or  less  extent  positively  hydrotropic.  Notice  what  is 
shown  in  Figure  135.  In  response  to  the  water  influence,  the 
roots  of  most  cultivated  plants  grow  deeper  in  the  soil  during 


Fig.  135.  —  An  experiment  to  show  the  effect  of  moisture  upon  the  direc- 
tion of  the  growth  of  roots.  The  box  containing  moist  sawdust  in  which  the 
Com  is  planted  has  a  bottom  of  wire  netting.  After  the  roots  grew  through 
the  meshes,  thus  coming  in  contact  with  dry  air,  they  changed  their  direction 
and  grew  along  the  bottom  of  the  box,  thus  keeping  in  contact  with  moisture. 
Adapted  from  Osterhout. 

a  dry  season  than  during  a  wet  season.  When  there  is  abun- 
dance of  moisture  in  the  soil,  Corn  roots  may  grow  within  2 
inches  or  less  of  the  surface,  but  are  3  inches  or  more  under  the 
surface  when  there  is  a  lack  of  moisture,  and  usually  penetrate 


152  ROOTS 

the  soil  to  a  greater  depth.  The  roots  of  Willows  and  Poplar? 
will  extend  long  distances  in  response  to  moisture.  When  these 
trees  grow  near  a  well,  their  roots  often  grow  down  the  sides  of 
the  well  until  the  water  is  reached.  In  seeking  water  and  air  the 
roots  of  trees  and  weeds  grow  into  drain  tiles  and  sewers,  often 
clogging  them. 

Stems,  in  general,  grow  toward  the  light,  while  most  roots 
shun  the  light.  Roots  are  said  to  be  negatively  heliotropic,  while 
stems  are  positively  heliotropic.  A  erotropism,  growth  toward  those 
regions  of  the  soil  where  air  is  more  plentiful,  Chemotropism, 
growth  toward  certain  substances,  and  Traumatropism,  growth 
away  from  injurious  bodies,  are  other  movements  of  roots. 


The  Soil  as  the  Home  of  Roots 

In  the  most  general  meaning  of  the  term,  the  soil  is  that  upper- 
most layer  of  the  earth's  crust  in  which,  by  means  of  their  root 
sj^stems,  plants  are  able  to  obtain  the  substances  necessary  for 
growth.  However,  in  agriculture,  the  term  soil  is  often  applied 
to  the  layer  which  is  tilled,  and  the  term  subsoil  to  that  which 
lies  beneath.  Although  the  term  soil  is  used  in  different  ways, 
we  usually  think  of  the  soil  as  extending  down  to  where  the 
dark  color  changes  to  a  light,  due  to  the  al3sence  of  humus.  The 
depth  of  the  soil  varies  greatly  in  different  localities,  ranging 
from  a  few  inches  to  several  feet. 

As  to  origin,  the  soil  is  fundamentally  pulverized  rock  of  which 
there  are  a  number  of  kinds,  such  as  granite,  limestone,  sandstone, 
shales,  etc.,  each  of  which  gives  some  special  property  to  the  soil. 
Various  agencies,  such  as  wind,  water,  ice,  chemicals,  tempera- 
ture variations,  and  plants  are  active  in  ])reaking  all  rocks  into  a 
pulverized  form.  They  may  be  very  finely  pulverized  into  clay, 
as  the  silicates  are,  or  left  in  the  form  of  fine  sand,  coarse  sand, 
or  gravel. 

The  rock  constituents  of  any  bit  of  soil,  even  of  the  finest  clays, 
are  exceedingly  various  in  size  and  shape  as  a  microscopical 
examination  shows.  The  irregularity  in  size  and  shape  makes 
it  impossible  for  the  particles  to  pack  closely,  and  thus  insures 
the  open  spaces  which  are  estimated  to  be  from  25  to  50  per  cent 
of  the  volume  of  cultivated  soils.  (Fig.  136.)  The  spaces  are 
exceedingly  important,  for  they  permit  the  circulation  of  water 


WATER,   AIR,   AND  HUMUS  OF  THE  SOIL 


153 


and  air  which  the  roots  and  micro-organisms  of  the  soil  must  have. 
Although  not  tightly  packed,  the  particles  adhere  to  each  other 
when  moist,  and  this  feature  and  the  weight  of  the  soil  eni?ble 
roots  to  obtain  a  firm  anchorage. 


Fig.  136.  —  Diagram  of  two  root  hairs  and  the  soil  arour  1  them.  The 
soil  particles  arc  shaded  and  the  light  a'.^a  around  each  soil  particle  repre- 
sents a  fihn  of  water.  The  large  light  area?  among  the  soil  particles  are  air 
spaces.     Modified  from  J.  G.  Coulter. 

Water,  Air,  and  Humus  of  the  Soil.  —  To  the  plant  the  water 
of  the  soil  has  two  important  functions.  First,  it  is  the  reservoir 
upon  which  the  plant  depends  for  water.  Second,  water  is  the 
solvent  in  which  the  soil  substances  become  dissolved  before 
entering  the  plant. 

The  amount  of  soil  water  varies  for  different  kinds  of  soils,  and 
for  the  same  soil  at  different  times.  Thus  garden  soil  rich  in 
humus  or  a  very  heavy  clay  soil  will  hold  two  or  three  times  as 
much  water  as  a  sandy  soil.  Just  after  a  heavy  rain  soils  are 
saturated  with  water,  that  is,  all  of  the  spaces  are  filled.  But 
much  of  this  water,  known  as  free  water,  gradually  sinks  away 
toward  the  center  of  the  earth  in  response  to  its  own  weight,  leav- 
ing the  pores  partially  empty.  The  water  then  remaining  in  the 
pores  consists  chiefly  of  capillary  water,  which  is  held  in  the  pores 
by  the  force  of  capillarity.  In  addition  to  the  capillary  water, 
which  does  not  respond  to  the  influence  of  gravity,  there  is  also 
the  hygroscopic  tvater,  which  remains,  after  the  capillary  water  is 
removed,  as  a  thin  film  around  each  particle  and  so  firmly  held 


154  ROOTS 

that  it  can  not  be  driven  off  except  by  the  appHcation  of  heat. 
The  driest  of  "air  dry"  soils  still  contain  considerable  hygroscopic 
water,  as  shown  by  their  loss  in  weight  when  they  are  further 
dried  in  an  oven. 

Plants  depend  mainly  upon  capillary  water,  although  in  some 
cases,  especially  in  soils  with  high  hygroscopic  power,  some  hygro- 
scopic water  may  be  available  to  the  plant.  When  soils  are  satu- 
rated, as  after  heavy  rains  or  in  bogs  and  swamps,  there  is  more 
water  present  than  the  plant  needs,  and,  besides,  the  air  which 
roots  must  have  is  driven  from  the  soil  pores.  Experiments  have 
shown  that,  in  general,  a  soil  is  best  adapted  to  plant  growth 
when  the  water  present  is  not  more  than  60  per  cent  of  the  amount 
required  for  saturation,  or,  in  other  words,  when  about  two-fifths 
of  the  pores  are  open  for  the  circulation  of  air. 

The  forces  which  resist  the  pulling  away  of  hygroscopic  and 
capillary  water  from  the  soil  particles  tend  to  keep  the  water 
equally  distributed.  Thus  as  water  is  lost  from  the  pores  in  the 
surface  soil,  either  by  evaporation  or  root  absorption,  it  is  replaced 
by  water  moving  up  from  below  through  the  force  of  capillarity. 
Consequently,  as  a  root  absorbs,  the  movement  of  water  toward 
it  from  all  around  enables  it  to  obtain  water  from  regions  several 
feet  away.  In  fact,  capillarity  has  been  known  to  raise  water  to 
a  height  of  10  feet  in  one  kind  of  soil.  Again,  in  hygroscopic 
water  the  thin  films,  which  are  like  stretched  rubber  around  the 
soil  particles,  are  connected  where  the  soil  particles  touch,  and  to 
compensate  the  greater  water  loss  one  film  may  have  over  others, 
there  is  such  a  movement  of  water  between  the  films  that  all  for 
a  considerable  distance  around  share  in  the  loss.  Thus,  due  to 
the  forces  of  capillarity  and  the  surface  tension  of  hygroscopic 
films,  soil  water  tends  to  move  to  the  point  where  it  is  being 
absorbed.  It  is  now  clear  why  the  soil  becomes  so  evenly  dry 
around  a  plant. 

Air  in  the  soil  is  necessary  for  the  respiration  of  roots  and 
micro-organisms.  It  is  also  of  use  in  oxidizing  poisonous  sub- 
stances which  result  from  the  decay  of  organic  matter  in  the  soil, 
so  that  their  poisonous  effects  on  roots  are  destroyed. 

Humus  consists  of  organic  matter  in  a  state  of  decomposition. 
When  only  partially  decayed  as  in  some  bogs  where  it  accumu- 
lates in  large  quantities,  it  forms  j)eat.  It  gives  to  soils  the  dark 
color  which  is  characteristic  of  good  soils,  such  as  loams  where  it 


SOIL  MICRO-ORGANISMS 


155 


occurs  well  mixed  with  sand  and  clay.  It  adds  to  the  soil  various 
organic  substances,  some  useful  and  some  harmful,  enables  the 
soil  to  retain  more  moisture,  makes  the  soil  light,  and  makes  the 
soil  a  suitable  place  for  micro-organisms  to  live. 

Soil  Micro-organisms.  —  The  soil  is  the  home  of  innumerable 
organisms,  some  plants  and  some  animals,  all  of  which  are  related 
to  soil  fertility.  They  are  of  three  kinds.  Fungi,  Bacteria,  and 
Protozoa.  These  organisms,  influencing  the  soil  fertility  and 
having  their  activities  in  turn  influenced  by  the  soil  conditions, 
add  to  the  complexity  of  soils,  which 
are   still   far  from   being  understood. 

Many  Molds  occur  in  the  soil,  where 
their  thread-like  filaments,  like  those 
of  Bread  Mold,  aid  in  breaking  up  the 
organic  matter  into  soluble  compounds. 
Besides  Molds,  there  are  other  kinds 
of  Fungi  which  act  on  the  soil  con- 
stituents, as  in  case  of  Toadstools 
which  invade  the  soil  with  their  root- 
like filaments.  Furthermore,  some 
Fungi  are  so  intimately  connected  with 
the  roots  of  some  plants  as  to  replace 
the  root  hairs.  In  this  case  the  Fun- 
gus weaves  around  the  root  a  close 
covering  of  filaments,  thus  forming 
with  the  root  the  structure,  known  as 
the  mycorrkiza,  in  which  the  filaments 
of  the  Fungus  absorb  water  and  solu- 
ble substances,  which  afterward  are 
transmitted  to  the  root.  {Fig.  137.)  Pines,  Beeches,  Oaks,  Blue- 
berries, and  Orchids  are  some  of  the  more  familiar  plants  in 
which  the  mycorhizas  occur.  Plants,  like  Blueberries,  are  so 
dependent  upon  Mycorhizas  that  they  can  not  be  grown  unless 
the  proper  Fungus  is  present  in  the  soil. 

The  Bacteria,  the  smallest  of  all  living  organisms  and  well 
known  in  connection  with  diseases  of  animals  and  plants,  are  ex- 
ceedingly abundant  in  soils,  often  many  millions  being  present 
per  cubic  centimeter  of  soil.  Like  the  Fungi,  the  Bacteria  of  the 
soil  are  dependent  in  their  development  upon  the  presence  of 
some  organic  food  material,  such  as  decaying  plant  or  animal 


Fig.  137. — A  Mycorrhizaon 
a  rootlet  of  the  Beech.  The 
felt-like  mass  of  mycelial 
threads  closely  enwraps  the 
root  tip,  extending  back  to 
beyond  the  hair  zone  and 
spreading  into  the  soil  like 
root  hairs.     After  Frank. 


156  ROOTS 

matter.  They  are  of  various  kinds  and  perform  various  func- 
tions, the  most  important  of  which  has  to  do  with  making 
nitrogen  available  for  root  absorption.  The  importance  of  the 
Bacteria  concerned  with  providing  available  nitrogen  in  the  soil 
is  due  to  the  fact  that  nitrogen  is  an  indispensable  constituent  of 
protoplasm,  and,  although  composing  four-fifths  of  the  air,  it  is 
only  available  when  it  occurs  in  the  form  of  a  soluble  salt  in  the 
soil  where  it  is  taken  in  through  the  roots.  To  maintain  in  the 
soil  an  adequate  supply  of  available  nitrogen,  which  is  constantly 
being  lost  by  the  removal  of  the  crop  and  through  drainage,  is  an 
important  problem  in  maintaining  soil  fertility.  The  Bacteria 
provide  available  nitrogen  in  two  ways.  First,  some  kinds  act 
on  the  ammonia  which  is  usually  abundant  where  there  is  humus, 
forming  nitrates,  which  are  soluble,  and  in  which  form  the  nitrogen 
is  available  to  our  higher  plants.  Second,  certain  kinds  of  Bac- 
teria use  the  free  nitrogen  of  the  air  in  forming  the  nitrogenous 
compounds  of  their  bodies,  which  after  death  release  these  com- 
pounds to  the  soil,  and  in  this  way  the  soil  has  its  nitrogen  in- 
creased. Thus  while  some  Bacteria  change  the  nitrogenous 
substances  already  present  in  the  soil  into  forms  which  can  be 
used  as  a  source  of  nitrogen  by  the  higher  plants,  these  forms,  by 
adding  nitrogenous  compounds  through  the  rapid  multiphcation 
and  early  death  of  their  bodies,  actually  increase  the  nitrogen 
content  of  the  soil,  and  for  this  reason  are  of  most  importance  in 
maintaining  the  soil  fertility.  Although  many  of  these  Bacteria 
which  fix  the  free  nitrogen  of  the  air  live  free  in  the  soil,  there  are 
some,  however,  which  live  in  the  roots  of  some  of  the  higher 
plants,  especially  in  those  of  Clover,  Alfalfa,  Beans,  and  other 
Legmnes.  In  this  case  they  live  in  the  nodules  formed  on  the 
roots,  and  the  relation  between  the  Bacteria  and  the  higher  plant 
is  said  to  be  one  of  syjnhiosis,  a  name  applied  to  such  an  intimate 
association  of  organisms.  In  this  case  both  organisms  are  bene- 
fitted; for  the  Bacteria  obtain  some  food  from  the  higher  plant 
and  the  latter  obtains  nitrogenous  compounds  from  the  dead 
bodies  of  the  Bacteria.     (Fig.  138.) 

However,  not  all  kinds  of  soil  Bacteria  are  so  indispensable, 
for  there  are  some  which  have  harmful  effects,  in  that  they  tend 
to  lessen  the  nitrogen  content  of  the  soil  by  so  thoroughly  de- 
composing the  nitrogenous  compounds  that  the  nitrogen  escapes 
from  the  soil  as  free  nitrogen  or  as  ammonia  gas.     Among  so 


SOIL  SOLUTION 


157 


many  kinds  of  soil  Bacteria,  breaking  up  compounds  in  different 
ways  to  secure  energy  and  food,  not  all  results  of  their  activi- 
ties in  the  soil  can  be  expected  to  be  desirable  ones. 

The  Protozoa,  which  are  small  one-celled  animals  of  which 
the  Amoeba  is  one  type,  are  abundant  in  rich  soils,  where 
they  are  thought  to  exert  a  harmful  in- 
fluence on  the  soil  fertility  by  feeding 
on  the  Bacteria.  The  evidence  for  this 
accusation  is  that  soils  are  more  fertile 
after  being  subjected  to  temperatures 
or  poisons  which  kill  the  Protozoa  but 
leave  the  Bacteria  unharmed. 

Soil  Solution.  —  The  soil  water  and 
the  various  mineral  matters  and  or- 
ganic substances  dissolved  in  it  consti- 
tute the  soil  solution.  The  dissolved 
organic  substances  are  of  use  to  the  soil 
micro-organisms,  but  it  is  mainly  water 
and  mineral  matters  that  higher  plants 
need  to  obtain  from  the  soil  solution. 
The  most  important  of  the  mineral  ele- 
ments for  crops  are  nitrogen,  -phosphorus, 
potassium,  sulphur,  calcium,  iron,  and 
rnagnesium.  These  occur  in  compounds  known  as  mineral  salts, 
which,  although  very  essential  to  plant  growth,  are  present  in 
very  small  quantities,  usually  constituting  less  than  one  per 
cent  of  the  best  of  soil  solutions.  Of  these,  nitrogen,  phos- 
phorus, and  potassium  are  in  most  demand  by  crops,  and  the 
ones  most  hkely  to  be  lacking.  Consequently,  in  maintaining 
soil  fertility,  the  chief  problem  is  to  conserve  and  restore  these 
elements.  The  value  of  artificial  fertilizers  and  manures  de- 
pends chiefly  upon  the  amount  of  these  elements  contained. 
In  most  soils,  iron,  sulphur,  and  magnesium  are  present  in 
sufficient  quantities.  Calcium  must  always  be  present  to 
neutralize  the  acids,  for  both  roots  and  soil  Bacteria  are  very 
sensitive  to  acids.  Calcium  is  added  to  the  soil  in  the  form  of 
lime  or  limestone.  On  the  other  hand,  when  soils  contain  too 
much  of  an  alkali,  such  as  sodium  carbonate,  plants  will  not 
do  well  until  the  condition  is  changed  hy  the  addition  of  gypsum 
or  some  other  substance  capable  of  breaking  up  the  alkali. 


Fig.  138.  —  Nodules  on 
the  roots  of  a  Pea.  Modi- 
fied from  Palladin. 


158 


ROOTS 


Each  of  the  mineral  salts  which  plants  require,  apparently, 
is  so  specially  related  to  the  nutrition  of  the  plant,  that  not  one 
of   them   can   be   omitted,    although   all   others  are   present   in 
suitable   quantities.     This  fact  is  dem- 
onstrated by  growing  plants  with  their 
roots  in  distilled  water  to  which  the  dif- 
ferent mineral  salts  can  be  added  in  such 
proportions  as  the  experiment  demands. 
When  the  salts  are  added  in  such  pro- 
portions that  the  solution  imitates  a  soil 
solution,  such  as  ordinary  spring  or  well 
water,  many  herbaceous  plants  are  able 
to  grow  in  it  till  they  have  flowered  and 
produced  seed.     In  fact,  aside  from  the 
lack  of  anchorage  and  having  to  supply 
their  roots  with  oxygen  from  the  shoot, 
4       2.      I     li     5         plants  may  do  almost  as  well  as  when 
Fig.  139.— Water  cultures  rooted  in  the  soil.     For  some  plants  the 
of  Buckwheat,  showing  ef-  water  culture  gives  good  results,  when 
the  salts  are  in  such  a  proportion  that 
2  liters  of  the  solu- 
tion contain  1  gram 
with  soda  instead  of  potash;    of  potassium  nitrate, 
4,  without  calcium;  5,  with-    ^  gram  of  iron  phos- 
out    nitrates    or    ammonia   p^ate,  1  gram  of  cal- 
cium sulfate,  and  I 
gram  of  magnesium  sulfate.     The  results  of 
omitting  some  of  these  salts  are  shown  in 
Figure  139. 

Root  Absorption.  —  For  the  process  of  os- 
mosis upon  which  the  entrance  of  water  into 
the  root  depends,  the  epidermal  cells  of  the  fig.  140.  — Root 
root  tip  are  especially  fitted.  By  means  of  hair  showing  the  thin 
the  root  hairs  they  have  a  large  surface  in  layer  of  protoplasm 

contact  with  the  soil  solution.     Having  thin  ^"^  ^^^^^  vacuole. 

„    ,  ,,  .  ,  .   ,     ,     •  ^1  After  Frank, 

cellulose  walls  agamst  wmich  then-  protoplasm 

is  spread  out  into  a  thin  lining,  root  hairs  afford  an  easy  entrance 

of  water  into  their  large  vacuoles.     {Fig.  I40.)     As  the  student 

already  knows  from  his  acquaintance  with  osmosis,  the  entrance 

of  water  into  the  epidermal  cell  depends  upon  the  concentration 


feet  of  the  lack  of  the  dif 
ferent    mineral   elements 

1,  with  all  the  elements 

2,  without    potassium;     3 


FACTORS   THAT   HINDER   ABSORPTION  159 

of  the  substances  in  solution  in  the  cell  sap.  Water  is  drawn 
into  the  root  hairs  only  when  the  density  of  the  cell  sap  is 
sufficient  to  exert  an  osmotic  force  that  overcomes  the  osmotic 
force  of  the  solution  without  and  the  forces  by  which  the  soil 
holds  on  to  the  water.  On  the  other  hand,  when  the  forces 
without  are  stronger  than  the  osmotic  force  of  the  cell  sap, 
then  water  will  be  drawn  out  of  the  root.  This  latter  condition, 
which  is  likely  to  be  disastrous  to  the  plant,  occurs  when  there 
is  an  excessive  amount  of  mineral  salts  in  the  soil  solution,  or 
when  the  soil  becomes  so  dry  that  the  forces  with  which  the 
soil  holds  on  to  the  water  become  so  great  as  to  overcome  the 
osmotic  force  of  the  cell  sap.  By  watering  plants  with  nutrient 
solutions  which  are  too  strong,  the  soil  solution  may  become  so 
concentrated  as  to  injure  the  plants. 

The  entrance  of  the  dissolved  mineral  salts  into  the  root  hairs 
depends  chiefly  upon  two  things:  First,  the  cell  membrane  must 
be  permeable  to  them.  Second,  the  membrane  being  permeable 
to  them,  they  pass  into  the  root  hairs  by  the  laws  of  diffu- 
sion. Thus,  if  a  salt  is  more  concentrated  without  than  within 
the  root  hairs,  it  passes  into  them  until  it  is  as  plentiful  within 
as  without.  Also  substances  may  diffuse  out  of  the  root  hairs 
when  more  concentrated  within  than  without.  Although  the 
movement  of  the  salts  may  be  influenced  in  rate  by  the  move- 
ment of  the  water,  experiments  show  that  the  amount  of  min- 
eral salts  which  enter  the  plant  is  quite  independent  of  the 
amount  of  water  absorbed.  However,  in  being  alive,  the  cell 
membrane  presents  some  features  not  found  in  connection  with 
dead  membranes.  One  peculiar  feature  is  that  it  can  alter 
its  permeability  from  time  to  time,  and  another  is  that,  al- 
though it  allows  many  substances  to  pass  in,  it  allows  very 
few  to  pass  out.  In  being  permeable  to  some  substances  and 
not  to  others,  roots  are  thereby  able  to  exercise  selective  ab- 
sorption, which  in  general  favors  the  entrance  of  the  more  useful 
substances,  although  roots  by  no  means  keep  out  all  harmful 
substances. 

From  the  epidermal  cells,  the  water  and  mineral  salts  pass 
through  the  cortex  to  the  xyleni  vessels  through  which  they  reach 
the  shoot.     {Fig.  HI.) 

Factors  that  Hinder  Absorption.  —  The  forces  concerned  in 
capillarity  and  surface  films  increase  as  the  water  of  the  soil  de- 


160 


ROOTS 


creases.  The}-  retard  absorption  and  may  become  so  great  as  to 
actually  prevent  it.  The  wilting  of  plants  when  the  soil  becomes 
dry  is  not  due  to  the  fact  that  there  is  no  water  in  the  soil,  but  to 
the  tact  that  the  roots  can  not  pull  the  water,  known  as  the  un- 
available water,  away  from  the  soil  particles.     It  has  been  found 


Fig,  141.  —  Lengthwise  section  through  a  root,  showing  the  way  the  water 
and  mineral  substances  of  the  soil  reach  the  vascular  bundles,  e,  epidermis 
with  root  hairs;  c,  cortex;  a,  endodermis;  p,  pericycle;  x,  xylem  of  vascular 
bundle.  The  arrows  indicate  the  way  the  water  and  dissolved  substances 
pass  to  the  vascular  bundles.     After  MacDougal. 

that  most  plants  wilt  when  the  soil  moisture  is  reduced  to  4.6  per 
cent  in  medium  fertile  garden  soil,  7.8  per  cent  in  sandy  loam,  and 
49.7  per  cent  in  peaty  soil.  From  these  figures  it  is  seen  that  the 
amount  of  unavailable  water  depends  much  upon  the  kind  of  soil. 
As  shown  in  the  table  below,  it  also  depends  much  upon  the  kind 
of  plant,  for  plants  differ  widely  in  their  ability  to  pull  water 
awa}^  from  the  soil  particles. 


UNAVAILABLE  WATER  FOR  DIFFERENT  PLANTS  IN  LOAM  SOIL 

Plant. 

Unavailable  water. 

Cabbage                            

Per  cent. 
5.8 

Corn                                        

5.9 

Oats                                              

6.2 

Asparagus                                             

7.0 

Lettuce                                                               .           

8.5 

Cucumber                                                                           .    .  . 

10.8 

Pondweed  (a  water  plant) 

24  8 

SUBSTANCES   GIVEN   OFF   BY   ROOTS  161 

On  the  other  hand,  when  the  soil  water  is  so  plentiful  that  air  is 
excluded  from  the  soil,  root  growth  is  retarded  and  absorption 
thereby  hindered.     For  this  reason  wet  lands  have  to  be  drained. 

In  retarding  growth  as  well  as  slowing  down  osmosis,  a  low 
temperature  of  the  soil  retards  absorption.  This  fact  is  related 
to  winter  killing,  which  is  sometimes  due  to  the  fact  that  the  roots 
can  not  absorb  water  from  the  cold  or  frozen  soil  about  them  as 
rapidly  as  it  is  lost  from  the  shoot. 

There  are  other  factors,  such  as  the  presence  of  alkalies  and 
certain  acids  in  the  soil  which  hinder  absorption  by  their  injurious 
effects  on  roots. 

Root  Pressure.  —  The  absorptive  action  of  roots  sometimes 
manifests  itself  in  forcing  water  through  the  stem,  acting  much 
like  the  pump  which  forces  the  water  through  the  city's  water 
mains.  This  pressure  exerted  by  roots  is  known  as  root  or  sap 
pressure,  and  is  one  cause  of  the  so-called  bleeding  of  plants  when 
they  are  injured.  In  most  plants  of  the  temperate  regions,  root 
pressure  is  only  evident  in  the  spring  when  the  plants  are  not 
losing  much  water  by  evaporation  and  are  gorged  with  sap. 
When  Grapes  are  pruned  in  the  spring,  they  usually  bleed  pro- 
fusely. A  vigorous  European  Grape  will  sometimes  bleed  a  liter 
per  day.     A  Maple  tree  may  exude  from  5-8  liters  in  a  day. 

Measurements  show  that  sap  pressure  is  often  several  pounds 
to  the  square  inch.  In  the  following  table  the  pressure  is  recorded 
in  millimeters  of  mercury  (760  millimeters  of  mercury  =  1  at- 
mosphere or  about  15  lbs.  of  pressure  to  the  square  inch). 

Red  Currant 358 

Sugar  Maple 1033 

Black  Birch 2040 

European  Grape S60 

Substances  Given  off  by  Roots.  —  Roots  give  off  carbon  diox- 
ide. The  milky  appearance  of  lime  water  in  which  roots  are 
grown  is  evidence  of  this  fact  (page  97).  In  the  soil  the  carbon 
dioxide  unites  with  the  water,  forming  carbonic  acid,  which  has 
an  important  dissolving  action  upon  the  soil  minerals.  This  fact 
is  demonstrated  by  the  etching  effect  roots  have  when  grown  in 
contact  with  the  surface  of  polished  marble. 

There  is  much  evidence  that  roots  also  give  off  oxidizing  en- 
zymes whereby  the  poisonous  substances  of  the  soil  are  oxidized 
to  harmless  forms. 


162  ROOTS 

It  has  been  deinonstrated  in  case  of  some  plants  that  roots  ex- 
crete poisonous  substances  which  tend  to  impede  further  root 
activit3^  These  deleterious  substances,  with  those  formed 
through  the  decomposition  of  roots  and  other  organic  matter,  may 
be  responsible  for  much  of  the  soil  sterility  that  is  so  commonly 
attributed  to  the  lack  of  the  necessary  mineral  salts.  In  fact, 
some  think  that  the  value  of  fertilizers  depends  mainl}^  upon  their 
neutralizing  effect  of  these  deleterious  substances.  The  improve- 
ment of  the  soil,  when  fields  are  allowed  to  lie  fallow,  is,  at  least, 
partly  due  to  the  disappearance  of  these  deleterious  substances 
through  oxidation.  It  seems  that  in  many  cases  the  deleterious 
substances  are  more  poisonous  to  the  roots  of  plants  of  the  same 
kind,  and  this  may  help  explain  the  value  of  crop  rotation. 

Water,  Air,  and  Parasitic  Roots 

Water  Roots.  —  When  branches  of  some  herbaceous  plants  are 
cut  off  and  set  in  water,  roots  develop  from  the  submerged  por- 
tion. Branches  of  the  Geranium  and 
Wandering  Jew  root  readily  in  water  and 
will  grow  for  a  long  time  in  ordinary 
river  or  well  water.  The  twigs  of  Willows 
Fig  142  —  Lemna  a  ^^^^  develop  water  roots  when  set  in 
floating  water  plant,  which  water.  Willows,  growing  on  the  edge  of 
has  only  water  roots,  ponds  and  streams,  develop  roots  which 
Slightly  magnified.  After  penetrate  the  soil  and  also  roots  which 
Stevens.  dangle  in  the  water.     There  are  a  number 

of  small  Seed  Plants,  like  the  Duckweeds,  which  float  on  the 
surface  of  the  water  and  have  no  roots  other  than  water  roots. 
{Fig.  U2.) 

Air  Roots.  —  Some  plants  depending  upon  soil  roots  also  de- 
velop air  roots.  The  brace  roots  of  Corn  are  at  first  air  roots  and 
later  enter  the  soil.  Some  climbing  plants,  hke  the  Poison  Ivy, 
develop  air  roots  which  attach  the  plant  to  the  support.  Many 
Orchids  and  some  plants  of  the  Pineapple  family  grow  supported 
on  other  plants  and  have  only  air  roots.  The  Tillandsia,  called 
Spanish  Moss,  although  not  a  Moss  at  all,  is  very  common  in 
southern  regions,  growing  on  the  branches  of  trees  with  its  roots 
dangling  in  the  air. 

Air  roots  differ  in  structure  from  soil  roots.     Air  roots,  unless 


PARASITIC    ROOTS  163 

they  are  growing  in  wet  shady  places,  are  not  in  a  good  position 
for  absorption.  Air  roots  of  cHmbers,  as  in  the  Poison  Ivy,  do  no 
absorbing,  and  serve  only  to  attach  the  plant  to  the  support. 
Those  air  roots  that  absorb  usually  have  no  root  hairs,  and  the 
absorbing  is  done  by  a  sponge-like  mantle  of  cells,  called  velamen, 
covering  the  root.  In  some  cases,  as  in  many  tropical  climbers, 
the  air  roots  reach  to  the  ground  or  to  cup-shaped  leaves  where 
water  is  obtained.  The  air  roots  of  the  Orchids  which  live  on 
damp  tree  trunks  or  rocks  of  tropical  countries  take  up  moisture 
when  there  is  rain  or  dew.  Such  plants,  called  epiphytes,  flourish 
without  the  assistance  of  soil  roots. 

Parasitic  Roots.  —  There  are  a  large  number  of  plants,  called 
parasites,  that  depend  upon  other  plants  for  food.  The  Dodder  is 
dependent  upon  other  plants 
for  its  food  and  obtains  it 
by  sending  roots  into  the 
plant  upon  which  it  is  grow- 
ing. Dodder  has  no  food- 
making  pigment  and  the 
young  seedling  soon  perishes 


unless    it    can    obtain    food  j^ 

from  some  other  plant.  The  ^ig.  143.  -  A,  Dodder  {Cuscuta  Euro- 
thread-like  seedlings  are  sen-  p^a)  Uving  on  a  Hop  Vine;  B,  diagram- 
sitive  to  touch  and  coil  about  matic  drawing  of  a  cross  section  of  the 
weeds,  Clover,  Alfalfa,  or  Hop  Vine  showing  the  roots  of  the  Dodder 
other  plants  which  they  mav  having  penetrated  the  tissues  of  the  Hop 
chance  to  hit  in  their  growth.  V^"^"  After  Kerner. 
If  the  plant  has  suitable  food,  then  the  Dodder  grows  roots  into 
its  tissues  and  absorbs  food  from  it.  Clover,  Flax,  and  Alfalfa 
are  attacked  in  this  way  and  much  injured  by  Dodder.  Dodder  is 
considered  a  destructive  weed,  and  seed  containing  only  a  little 
Dodder  seed  is  undesirable  for  seeding.     {Fig.  143-) 

The  Mistletoe  lives  upon  trees,  the  roots  penetrating  the 
branches  and  withdrawing  the  necessary  foods.  Many  plants, 
such  as  the  Beech  Drop,  Broom  Rape,  etc.,  live  on  the  roots  of 
other  plants. 

Propagation  by  Roots 

The  production  of  new  plants  from  seeds,  stems,  leaves,  or  roots 
is  called  plant  propagation.  Since  roots  readily  produce  adven- 
titious buds  which  can  develop  into  new  plants,  they  are  much 


164 


ROOTS 


used  in  the  propagation  of  some  kinds  of  plants.  For  example, 
Sweet  Potatoes,  wliich  rarely  produce  seed,  are  propagated  by 
means  of  the  shoots  whicli  develop  from  the  fleshy  roots.  The  roots 
are  planted  early  in  the  spring  in  specially  prepared  beds,  usually 
hot  beds,  where  they  develop  buds  which  grow  into  stems  bear- 
ing leaves  and  roots,  as  shown  in  Figure  144-  These  young  plants 
(slips)  are  broken  loose  from  the  potato  and  planted  in  the  field 
after  all  danger  of  frost  is  passed.  The  abundance  of  stored  food 
enables  each  potato  to  produce  many  slips. 


Fig.  144.  —  Sprouting  of  the  Sweet  Potato.  A,  potato  with  sprouts  in 
different  stages  of  development.  B,  sprout,  or  sUp,  broken  loose  from  the 
potato  and  ready  to  be  set  out. 

From  the  roots  of  the  Red  Raspberry  ^  and  some  Blackberries, 
new  stems  called  suckers  grow  up.  These  suckers  with  a  small 
portion  of  the  parent  root  are  used  in  starting  new  plantations. 
The  larger  roots  of  the  Raspberry  and  Blackberry  are  often  dug 
up  in  the  fall,  cut  into  pieces,  and  stored  until  spring  when  they 
are  planted  in  the  field.  From  these  root  segments  new  plants 
are  produced.  Roses  are  often  propagated  by  root  cuttings. 
When  plants  can  be  propagated  either  by  root  cuttings  or  by 
seed,  it  is  generally  better  to  use  cuttings,  because  plants  obtained 
from  cuttings  usually  grow  faster  and  are  more  likely  to  be  like  the 
parent  plant  than  they  are  when  grown  from  seed. 

1  Raspberries.  Farmers'  Bulletin  213,  U.  S.  Dept.  Agr.  Culture  of  Small 
Fruits.  Bulletin  105,  Oregon  Agr.  Exp.  Sta.  Dewberry  growing.  Bulletin 
136,  Colorado  Agr.  Exp.  Sta. 


PROPAGATION   BY   ROOTS  165 

Some  trees,  like  the  Black  Locust  and  Silver-leaved  Poplar, 
are  troublesome  because  of  the  readiness  with  which  sprouts 
spring  up  from  the  roots.  Some  of  the  weeds,  such  as  Canada 
Thistle  and  Field  Sorrel,  spread  by  developing  new  plants  from 
buds  on  their  spreading  roots.  When  the  Dandelion  is  cut  off 
several  inches  under  the  ground,  it  often  grows  up  again  from 
the  portion  of  the  root  left  in  the  ground. 


\/ 


^•^•< 


CHAPTER   IX 

STEMS 

Characteristic  Features  and  Types  of  Stems 

The  stem,  usually  consisting  of  trunk  and  branches,  is  the  fun- 
damental part  of  the  shoot.  Upon  the  stem  the  other  structures 
of  the  shoot,  such  as  leaves,  flowers,  and  fruit,  depend  for  their 
support  in  the  air  and  sunlight  —  the  position  most  favorable  for 
leaf-activity,  pollination,  and  scattering  of  seed  and  fruit. 

Roots,  stems,  and  leaves  are  intimately  related  in  their  activi- 
ties, and  the  efficiency  of  one  affects  the  efficiency  of  the  others. 
The  productivity  of  most  of  the  cultivated  plants  depends  not 
only  upon  a  good  root  system,  but  also  upon  a  good  stem  system. 
In  some  plants,  such  as  Beets,  Turnips,  Radishes,  Lettuce,  and  a 
few  others  which  have  very  short  stems  during  much  of  their  life, 
not  so  much  importance  is  attached  to  the  stem,  but  even  these 
plants,  in  order  to  complete  their  life  cycle,  must  eventually 
develop  stems  upon  which  to  bear  flowers  and  seeds.  Among 
such  plants  as  the  trees  and  grains,  the  stem  is  very  important. 
The  value  of  a  tree  for  shade,  lumber,  or  fruit  depends  largely 
upon  the  character  of  the  stem.  Likewise,  a  Corn  or  Wheat  plant 
with  a  well  developed  stem  is  able  to  produce  larger  ears  or  a 
better  head  than  a  plant  with  a  stem  poorly  developed. 

In  comparing  stems  with  roots  the  following  things  may  be 
stated.  First,  stems  bear  leaves  and  flowers,  while  roots  do  not. 
Second,  stems  are  divided  into  nodes  and  internodes  but  roots  are 
not.  Third,  stems  branch  at  the  nodes,  while  in  roots  branches 
arise  anywhere.  Fourth,  in  stems  pith  is  nearly  always  present, 
while  in  roots  it  is  usually  absent. 

The  nodes  are  the  narrow  zones,  often  more  or  less  swollen,  at 
which  the  leaves  and  buds  as  well  as  the  branches  arise.  The 
internodes  arc  the  zones  between  the  nodes.  The  division  of  the 
stem  into  nodes  and  internodes  is  quite  noticeable  in  the  stems  of 
Corn  and  other  Grasses,  where  the  nodes  divide  the  stem  into 
distinct  segments.     By  the  elongation   of   the  internodes,    the 

166 


BRANCHING   OF   STEMS 


167 


leaves  are  separated  and  better  exposed  to  the  light.  If  the  in- 
ternodes  are  short,  as  in  the  stem  of  the  Dandelion,  the  leaves  are 
much  crowded.  Also  in  such  plants  as  Beets,  Radishes,  Turnips, 
and  Lettuce  the  stem  at  first  has  short  internodes  and  the  leaves 
are  much  crowded. 

On  the  ends  of  branches  as  well  as  in  the  axils  of  leaves,  occur 
the  buds  which  have  much  to  do  with  the  growth  of  stems.  The 
stem  elongates  by  the  development  .  j. 

of  new  nodes  and  internodes  from 
the  terminal  buds,  while  branches 
develop  from  the  buds  occurring  in 
the  axils  of  the  leaves. 

Branching  of  Stems.  —  Since 
branches  develop  from  the  buds 
located  in  the  axils  of  the  leaves, 
the  arrangement  of  branches  tends 
to  follow  the  leaf  arrangement. 
Plants  having  two  leaves  at  a  node 
and  on  opposite  sides  of  the  stem, 
as  in  the  Maple,  tend  to  have 
branches  with  the  opposite  arrange- 
ment. Likewise,  plants  with  leaves 
occurring  one  at  a  node  and  on 
alternate  sides  of  the  stem  tend  to 
have  the  alternate  arrangement  of 
branches,  as  Elms  illustrate. 

The  amount  of  branching  varies 
much  among  plants.  Among  herba- 
ceous plants  the  stems  of  many  of 
the  Grasses  branch  very  little  and 
are  called  simple  stems,  while  in 
some  plants,  as  Clover  and  Alfalfa 
illustrate,  there  is  very  much  branching.  Branching  reaches  its 
maximum  among  the  trees,  where  often  there  is  branching  and 
rebranching  until  the  youngest  branches  are  so  numerous  and 
small,  as  in  the  Elms  and  Birches,  that  the  tree  may  be  some- 
what brush-like  in  appearance. 

Branching  is  directly  related  to  leaf  display,  for  it  not  only 
enables  the  plant  to  bear  more  leaves,  but  makes  a  better  exposure 
to  sunlight  possible.     Branching  is  also  related  to  flower  and  fruit 


Fig.  145.  —  Pines,  showing 
the  exourrent  type  of  stem. 
After  Fink. 


168 


STEMS 


production,  for  a  well  branched  tree  can  produce  more  flowers  and 
fruit  than  one  that  is  less  branched,  provided  the  food  supply  is 
sufficient.  In  plants  used  for  forage,  such  as  Clover  and  Alfalfa, 
the  amount  of  hay  produced  by  a  plant  depends  largely  upon  the 
extent  of  branching. 

In  some  plants,  as  in  the  Pine  shown  in  Figure  145,  the  stem 
system  consists- of  a  main  axis  and  many  lateral  branches,  forming 
r-^ — -  what  is  known  as  the  excur- 

rent  type  of  stem.    In  others, 
.     .  as    in    the    Elm    shown    in 

Figure  I46,  the  main  stem  is 
divided  into  two  or  more 
branches,  which  are  soon  lost 
in  numerous  branches,  form- 
ing the  deliquescent  type  of 
stem.  Among  fruit  trees  and 
forest  trees,  there  is  so  much 
difference  in  habits  of  branch- 
ing that  many  kinds  of  trees 
can  be  identified  by  their 
branching  habit. 

Work  Done  by  Stems.  — 
There  are  four  important 
functions  of  stems.  They 
support  the  aerial  structures, 
conduct  materials,  make 
food,  and  serve  as  regions  of 
storage. 

The  su]) porting  function 
consists  in  carrying  the 
weight  of  the  leaves,  flowers, 
and  fruit,  and  in  elevating 
them  to  a  position  most  favorable  for  performing  their  func- 
tions. There  is  strong  competition  among  plants  for  light,  and 
it  is  through  the  elongation  of  the  stem  that  plants  lift  their 
leaves  higher  in  the  air  and  often  escape  the  shade  of  neigh- 
boring plants.  Some  plants,  such  as  the  Grape,  Poison  Ivy, 
Morning  Glory,  Beans,  and  Peas,  which  have  weak  stems,  secure 
better  light  by  climbing  a  support,  such  as  a  wall,  fence,  or  the 
stems  of  other  plants. 


Fig.  146.  — Elm  tree,  showing  deli- 
quescent type  of  stem. 


WORK   DONE   BY   STEMS  169 

As  a  Conductive  structure  the  stem  occupies  an  important  posi- 
tion, for  through  it  the  leaves  and  roots  exchange  materials. 
Consequently,  the  vascular  bundles,  forming  a  continuous  con- 
ductive system  from  roots  to  leaves,  are  prominent  structures  in 
stems.  Through  the  conductive  system  the  leaves  receive  water 
and  mineral  salts  from  the  soil  and  the  roots  receive  the  food  made 
in  the  leaves.  For  this  reason  any  injury  to  the  stem,  such  as 
girdling,  which  severs  the  conductive  system  is  likely  to  seriously 
injure  the  plant.  In  fact,  girdling  is  a  common  method  employed 
in  killing  trees. 

In  the  nianufadure  of  plant  foods  stems  may  assume  consider- 
able importance,  although  seldom  so  much  as  leaves,  which  have 
food-making  as  their  primary  function.  Being  well  exposed  to 
light  and  well  provided  with  chlorophyll,  leaves  are  especially 
adapted  to  carry  on  jjhotosynthesis  —  the  process  by  which  food  is 
manufactured.  However,  any  portion  of  a  plant  containing  chlo- 
rophyll to  which  sunlight  and  air  are  accessible-  can  make  food, 
and  the  stems  of  practically  all  plants  that  make  their  own  food 
have  some  portions  that  are  green  and,  therefore,  able  in  some  de- 
gree to  carry  on  photosynthesis.  For  example,  the  young  twigs 
of  trees  are  almost  as  green  as  the  leaves  and  no  doubt  make  con- 
siderable food.  As  the  twigs  grow  older,  the  green  layer  is  cov- 
ered by  bark,  which  excludes  the  light  that  is  necessary  for 
photosynthesis.  In  the  Box-elder,  Sassafras,  and  some  other 
trees,  not  only  the  young  twigs  but  portions  of  the  older  branches 
are  green,  and  probably  able  to  make  food.  In  most  of  our 
short-lived  plants,  such  as  Corn,  Sorghum,  Kafir  Corn,  Tomatoes, 
Melons,  Clover,  Alfalfa,  Beans,  etc.,  the  entire  stem  is  green  and 
able  to  carry  on  photosynthesis.  In  some  plants,  such  as  the 
Cacti,  which  have  no  leaves,  all  of  the  food  must  be  made  by  the 
stem.  In  the  garden  Asparagus  the  leaves  are  scale-like  and  food 
is  made  chiefly  by  the  stem  and  its  many,  small,  lateral  branches. 
Some  plants  which  have  scale-like  leaves,  have  green  lateral 
branches  so  expanded  as  to  resemble  leaves,  as  the  Smilax 
(Myrsiphyllum),  common  in  greenhouses,  illustrates.  Such 
branches  are  called  Cladophylls.     (Fig.  147.) 

As  to  the  storage  function  of  stems,  there  is  much  difference 
among  plants,  but  in  nearly  all  stems  there  is  some  accumulation 
of  substances,  such  as  water,  sugars,  and  starch.  During  the  wet 
season  the  stems  of  some  Cacti  take  up  large  amounts  of  water, 


170 


STEMS 


which  supply  the  plant  during;  the  dry  season.  In  the  stems  of 
Sorghum  and  Sugar  Cane,  so  much  sugar  is  accumulated  and  re- 
tained that  these  plants  are  grown  for  the  sugar  which  they  afford. 
In  the  stems  of  trees  much  food  is  stored  in  the  form  of  starch, 

and  when  transferred  to  grow- 
ing regions  during  early  spring, 
it  is  changed  to  sugar,  in  which 
form  it  occurs  in  solution  in 
the  sap  of  the  tree.  The  so- 
called  maple  sap  obtained  from 
the  Sugar  Maple  is  a  good  illus- 
tration of  sap  which  contains 
much  stored  food  in  the  form 
of  sugar.  In  early  spring  be- 
fore the  leaves  appear,  the  trees 
are  so  gorged  with  sap  that  it 
can  be  drawn  off  by  boring  into 
the  wood  and  inserting  spiles. 
This  sugar  comes  from  reserve 
food  accumulated  when  the 
leaves  are  active,  and  serves  as 
a  supply  for  the  growth  of  new 
foliage  at  the  beginning  of  the 
growing  season. 

Some  stems,  notably  those  of  the  Irish  Potato,  contain  large 
amounts  of  starch  on  account  of  which  they  are  valuable  for  food. 
Another  tuber-like  stem  similar  to  that  of  the  Irish  Potato  is  pro- 
duced by  the  Jerusalem  Artichoke  —  a  plant  of  the  Sunflower 
type  and  often  grown  on  account  of  the  food  value  of  its  under- 
ground tubers. 

Many  of  the  early  spring  plants,  such  as  Spring  Beauty,  Dutch- 
man's Breeches,  Wind  Flower,  some  Violets,  and  many  other 
plants  having  a  supply  of  food  at  hand  can  spring  up  quickly, 
flower,  and  accumulate  another  supply  of  food  before  the  sunlight 
is  excluded  by  the  forest  foliage.  Such  plants,  being  seen  only 
in  April  or  early  May,  have  what  is  called  the  vernal  habit,  i.e  , 
they  live  their  life  cycle  in  the  spring  of  the  year.  The  food 
reserve  of  stems  has  much  to  do  with  the  vernal  habit. 

Classes  of  Stems.  —  There  are  many  ways  in  which  stems  may 
be  classified.     Stems  are  classified  as  monocotyledonous  or  dicoty- 


FiG.  147.  —  A  branch  of  Myrsiphyl- 
lum,  showing  the  cladophylls  (a),  and 
the  scale-hke  leaves  (b). 


CLASSES   OF  STEMS  171 

ledonous,  according  to  whether  or  not  they  belong  to  Monocoty- 
ledons or  Dicotyledons.  However,  it  is  more  in  structure  than 
in  external  characters  that  these  two  types  of  stems  present 
important  differences. 

As  to  hardness  stems  are  often  classified  as  either  herbaceous  or 
woody.  Stems  that  are  typically  herbaceous,  like  those  of  Clover, 
Alfalfa,  Tomatoes,  and  others  which  develop  very  little  woody 
tissue,  are  soft  and  short-lived,  usually  living  but  one  year.  It  is 
among  trees,  where  the  amount  of  woody  tissue  reaches  its  maxi- 
mum, that  the  best  examples  of  woody  stems  occur.  However, 
between  herbaceous  and  woody  stems  there  is  no  distinct  line  of 
division,  for  most  herbaceous  stems  are  woody  in  their  older 
regions  and  all  woody  stems  are  herbaceous  in  their  younger 
regions.  The  terms,  herbaceous  and  woody,  refer,  therefore,  to 
the  amount  of  woody  tissue  present,  and  not  to  the  presence  or 
absence  of  it. 

As  to  length  of  life  stems  may  be  classified  into  annuals,  bien- 
nials, and  perennials.  Annual  stems  live  but  one  growing  season. 
The  stems  of  most  herbaceous  plants  are  annuals,  dying  down  to 
the  ground  either  before  or  after  frost  comes,  as  in  case  of  most 
vegetables,  weeds,  and  Grasses.  But  annual  stems  and  annual 
plants  must  not  be  confused,  for  many  plants,  hke  Alfalfa,  Quack 
Grass,  and  Canada  Thistle,  which  live  many  years,  thus  being 
perennial  in  habit,  have  annual  stems  which  grow  up  in  the  spring 
and  die  down  in  the  fall.  When  the  plant  is  annual,  roots,  stem, 
and  all  other  parts  die  at  the  end  of  the  growing  season,  and 
the  plant  must  be  started  anew  from  seed. 

In  plants,  such  as  Turnips,  Carrots,  and  Beets,  which  require 
two  years  to  complete  their  Hfe  cycle,  and  are,  therefore,  known 
as  biennials,  the  stem  remains  short  during  the  first  growing  season, 
forming  a  mere  crown  at  the  top  of  the  root.  During  the  second 
growing  season,  stems  develop  which  bear  flowers  and  seeds, 
and  then  the  entire  plant  dies.  In  some  biennials,  as  Cabbage 
and  Rape  illustrate,  the  stem  is  prominent  during  the  first  season, 
although  it  elongates  much  more  during  the  second  season  in 
preparation  for  bearing  flowers  and  seeds,  as  shown  in  Figure  I48. 
In  Red  Clover,  Sweet  Clover,  and  many  weeds  with  the  biennial 
habit,  the  portion  of  the  stem  known  as  the  crown  is  biennial, 
while  the  branches  arising  from  the  crown  are  annuals. 

Perennial   stems,  so   described  because   they  live   year  after 


172 


STEMS 


year,  are  typical  of  shrubs  and  trees,  although  thej^  occur  among 
herbaceous  plants,  notably  in  the  Ferns,  Sedges,  and  Grasses 
where  the  underground  stem,  which  is  well  protected  by  a 
covering  of  earth,  is  able  to  persist  for  many  years. 

As  to  position  stems  are  clas- 
sified into  aerial,  submerged, 
and  underground.  Submerged 
stems  are  of  least  importance, 
being  characteristic  of  plants 
which  grow  in  lakes  or  slug- 
gish streams,  where  the  plant 
is  often  supported  by  the 
buoyant  power  of  the  water 
rather  than  by  its  stem  sys- 
tem. Aerial  stems  are  of 
most  importance  to  us,  al- 
though there  are  some  valu- 
able underground  stems. 


Fig.  148.  —  Two  stages  in  the  development  of  a  Cabbage  plant.  A, 
plant  at  the  beginning  of  the  second  season's  growth  with  flowering  stem 
pushing  out  of  the  head.  B,  Cabbage  plant  in  flower  near  the  end  of  the 
second  growing  season,     a,  scars  left  by  the  falling  of  the  leaves  of  the  head. 


Aerial  Stems.  —  Most  of  our  cultivated  plants  as  well  as  most 
weeds  have  aerial  stems.  Since  aerial  stems  keep  above  ground, 
they  are  best  adapted  to  expose  leaves  to  the  air  and  sunlight. 
Aerial  stems  may  be  erect,  prostrate,  or  climbing. 


AERIAL   STEMS  173 

The  erect  stem  is  the  common  type  of  aerial  stem,  and  is  best 
adapted  for  leaf  display.  Having  the  erect  position,  it  can 
branch  and  display  leaves  on  all  sides,  and  by  elongation  can  lift 
its  leaves  above  the  shade  of  other  plants. 

Erect  stems  show  a  striking  contrast  to  primary  roots  in  the 
way  they  respond  to  gravity  and  light.  While  the  main  axis  of 
primary  roots  is  positively  geotropic  and  negatively  heliotropic, 
the  trunk  of  erect  stems  behaves  in  the  opposite  way,  thus  grow- 
ing away  from  the  center  of  the  earth  and  toward  the  light.  But, 
like  lateral  roots,  the  branches  of  the  shoot  tend  to  take  a  hori- 
zontal or  plagiotropic  position,  in  which  they  appear  indifferent  to 
both  light  and  gravity.  However,  this  indifference  to  gravity 
and  light  on  the  part  of  the  branches  of  the  shoot  seems  to  depend 
upon  influences  exerted  liy  other  parts  of  the  stem;  for,  if  the 
upper  part  of  the  shoot  is  removed,  then  the  horizontal  branches 
remaining  show  a  strong  tendencj^  to  become  erect. 

Erect  stems,  being  wholly  self-supporting  and  much  exposed 
to  winds,  surpass  other  stems  in  amount  of  strengthening  tissue 
developed.  From  this  type  of  stems  where  woody  tissue  reaches 
its  maximum  development,  as  in  trees,  we  obtain  timber.  How- 
ever, erect  stems  are  not  always  sufficiently  strong  to  endure  the 
strains  to  which  they  are  exposed,  as  is  well  known  in  case  of 
grains  where  the  so-called  ''  lodging  "  often  occurs. 

In  size  erect  stems  surpass  all  others.  The  most  remarkable 
erect  stems  are  those  of  the  giant  Sequoias,  one  of  which  is 
shown  in  Figure  149.  These  giant  trees,  which  are  natives  of 
the  western  mountains,  may  attain  a  height  of  400  feet,  a  cir- 
cumference of  more  than  100  feet,  and  live  to  ])e  more  than  4000 
years  old. 

The  prostrate  stems  of  Pumpkins,  Melons,  Squashes,  Cucumbers, 
Strawberries,  and  Sweet  Potatoes  are  well  known  to  the  student. 
They  are  not  strong  enough  to  maintain  an  erect  position,  and 
lie  stretched  upon  the  ground.  Prostrate  stems  are  common 
among  such  weeds  as  the  Five-fingers  and  Spurges.  Some  plants, 
as  Crab  Grass  and  Buffalo  Grass  illustrate,  have  both  erect  and 
prostrate  stems.  In  this  case  the  erect  stems  arise  as  branches 
from  the  prostrate  ones. 

The  prostrate  position  is  not  a  good  one  for  leaf  display;  for 
leaves  can  be  displayed  only  on  the  upper  side  and  not  all  around 
as  in  erect  stems.     Prostrate  stems  are  also  not  able  to  escape 


174 


STEMS 


the  shade  of  surrounding  plants  and,  therefore,  thrive  best  where 
erect  plants  are  few,  as  on  sandy  beaches  or  river  banks. 

However,  prostrate  stems  have  some  distinct  advantages  over 
erect  ones.     Since  they  do  not  have  to  support  much  weight,  they 


I'l,,.    1  I'.i.        One  of  the  .tiiant  t ivcs  of  t  lie  \\'i'8t,  called 
"LfVi/j/Ay  Cliaut."     From  Fuicstrv  Bulletin. 

can  grow  very  long  and  slender  without  developing  much  strength- 
ening tissue.  Furthermore,  they  are  not  so  much  exposed  to  loss 
of  water  by  evaporation  as  erect  stems  are.  The  prostrate  posi- 
tion is  also  more  favorable  for  vegetative  propagation,  for  as  the 


CLIMBING   STEMS 


175 


stems  grow  over  the  ground,  the  nodes  may  produce  roots  from 
their  lower  and  stems  from  their  upper  surface,  and  thus  new 
plants  are  started  which  become  independent  by  the  death  of 
the  parent  stem.  This  method  of  propagation  is  common  in  the 
Strawberry  where  the  prostrate  stems,  known  as  runners,  pro- 
duce roots  at  their  tips  and  start  new  plants  which  soon  become 
independent  by  the  death  of  the  runner.  {Fig.  150.)  Some  of 
the  Grasses  and  weeds  are  able  to  spread  very  rapidly  by  this 
method  of  propagation. 

This  disposition  of  nodes  to  grow  roots  and  start  new  plants  is 
an  important  feature  in  the  propagation  of  plants.     Not  only  the 


Fig.  150.  —  Prostrate  stem  (runner)  of  the  Strawberry  producing 
new  plants  at  the  nodes. 


nodes  of  prostrate  stems  will  do  this,  but  the  nodes  of  most  stems 
are  able  to  produce  roots  as  well  as  branches  and  leaves,  if  placed 
in  proper  conditions.  Much  use  is  made  of  this  feature  in  prop- 
agating many  of  our  useful  plants  as  we  shall  see  later. 

Climbing  Stems.  —  Some  familiar  examples  of  climbing  stems 
are  those  of  the  Pea,  Grape,  Hop,  Woodbine,  Poison  Ivy,  and 
Morning  Glory.  Climbing  stems,  like  prostrate  stems,  grow  long 
and  slender,  and  are  not  strong  enough  to  support  themselves  in 
an  erect  position.  They  raise  themselves  into  the  light  by  climb- 
ing a  support,  such  as  a  fence,  wall,  or  some  erect  plant.  Some 
kinds  of  Beans  having  climbing  stems  are  often  planted  with  the 
Corn,  so  that  they  may  have  the  Corn  stems  for  support,  or  when 
planted  alone,  each  plant  is  provided  with  a  stake  for  a  support. 
Sweet  Peas,  Hops,  and  most  Grapes  are  other  familiar  plants  re- 
quiring supports.  The  Woodbine  and  some  wild  Grapes  are 
quite  notable  climbers,  often  climbing  to  the  tops  of  tall  trees. 


176 


STEMS 


{Fig.  151.)     Many  of  the  most  notable  climbers  are  in  the  trop- 
ical regions. 

Climbing  stems  have  no  more  space  for  the  display  of  leaves 
than  prostrate  stems  have,  because  one-half  of  the  space  for  leaf 
display  is  cut  off  by  the  support;  but  the  climbing  position  is 
much  better  than  the  prostrate  position  for  escaping  the  shade 

of  other  plants. 

One  interesting  feature 
of  climbing  plants  is  their 
different  ways  of  climb- 
ing a  support.  The  Bean, 
Morning  Glory,  and  Hop 
climb  by  twining  around 
the  support.  They  are 
called  twiners.  These 
plants  can  not  climb  a 
wall,  for  they  must  have 
a  support  which  they  can 
wrap  about.  {Fig.  152.) 
The  Sweet  Pea  and  Grape 
Vine  illustrate  climbing 
by  means  of  tendrils 
which  hook  about  the  sup- 
port.  Tendrils  are  usually 

^  ^•h.lit-.... ^^^gJ^tfjMI y i^^^BB^^B    modified  leaves  or  stems, 
pt^'^'Tj^lj^P^^ll^m^^^^^^^l    although     sometimes     of 

doubtful  origin.  {Fig. 
153.)  In  some  tendril 
chmbers,  as  in  the  Japan 
Ivy,  the  tendrils  have 
swollen  ends  which  flatten 
against  a  wall  or  other 
supports,  where  they  se- 
crete a  mucilaginous  substance  by  which  they  are  able  to  hold 
on  tenaciously.  In  case  of  the  English  Ivy,  the  plant  is  held 
to  the  wall  by  roots  which  are  as  efficient  as  tendrils.  The 
Virginia  Creeper  climbs  by  means  of  both  roots  and  tendrils.  In 
being  able  to  climb  vertical  walls  of  stone  or  brick,  the  Ivies 
are  well  adapted  for  wall  vines  for  which  they  are  much  used. 
{Fig.  154.) 


Fig. 


151.  —  A  Grape  Vine  climbing 
a  dead  Elm  tree. 


UNDERGROUND  STEMS 


177 


Underground  Stems.  — ■  The  Potato,  Onion,  and  Artichoke  are 
famihar  examples  of  underground  stems.  Many  of  the  plants 
grown  in  the  greenhouse  and 
on  the  lawn  for  decoration, 
such  as  the  Lilies,  Hyacinth, 
Tulip,  Crocus,  etc.,  have  un- 
derground stems.  This  type 
of  stem  is  common  among 
plants  with  the  vernal  habit. 
Many  of  our  useful  Grasses, 
as  Red  Top,  Kentucky  and 
Canada  Blue  Grass,  Orchard 
Grass,  and  others  have  peren- 
nial subterranean  stems  from 
which  aerial  stems  are  sent  up 
each  year.  Grasses  of  this 
type  live  many  years  and  are 
the  Grasses  which  produce 
our  permanent  pastures. 
Grasses  of  this  type  are  also 
chosen  for  lawns,  because  their 
spreading  underground  stems 
produce  a  compact  sod  and 
send  up  a  thick  aerial  growth. 
Quack  Grass,  Johnson  Grass, 
some  Morning  Glories,  Poi- 
son Ivy,  and  many  other 
weeds  have  underground 
stems,  and  it  is  due  to  this 
feature  that  such  weeds  are 
hard  to  eradicate.  Cutting 
off  the  aerial  stems  of  these 
weeds  does  not  kill  the  plant; 
for  the  underground  portion 
still  lives  and  is  able  to  send 
up  more  aerial  stems. 

Underground  stems  are  least  adapted  for  displaying  leaves  and 
bearing  flowers,  and  they  must  either  produce  leaves  and  flower 
stalks  long  enough  to  reach  above  ground  or  grow  branches  which 
become  aerial  stems  upon  which  the  leaves,  flowers,  and  fruit  are 


Fig.  152.  —  Morning  Glory  twining 
around  a  Corn  stalk. 


178 


STEMS 


borne.     In  most  cases  aerial  stems  are  produced,  and  the  leaves 
of  the  underground  stem  are  mere  scales. 

Although  the  underground  stems  are  the  least  adapted  for  leaf 
display,  they  have  some  advantages  that  aerial  stems  do  not  have. 


Fig.  153. 


Smilax  climbing  over  bushes  by  means  of  tendrils. 
After  Kerner. 


They  are  much  less  exposed  to  drying  and  freezing,  and  escape 
being  pastured  off  by  stock.  They  are  safe  places  for  the  storage 
of  food,  and  most  underground  stems  do  have  much  reserve  food, 
which  is  used  in  the  growth  of  new  aerial  shoots  at  the  opening  of 
each  growing  season.  Herbaceous  plants  are  able  to  persist  for 
many  years,  if  they  have  an  underground  stem  from  which  new 
shoots  may  arise  each  year.  In  other  words,  an  underground 
stem  is  one  of  the  features  that  makes  it  possible  for  herbaceous 
plants  to  be  perennials.  The  underground  position  is  an  advan- 
tageous one  for  vegetative  propagation,  because  not  only  are  the 
nodes  favorably  located  for  establishing  roots,  but  the  supply  of 
reserve  food  and  protection  from  drying  and  freezing  makes  it 
possible  for  even  small  segments  of  underground  stems  to  live 
and  develop  separate  plants.  When  an  underground  stem  like 
that  of  Quack  Grass  is  hoed  to  pieces,  each  segment,  if  it  has  a 


UNDERGROUND  STEMS 


179 


node,  will  develop  a  new  plant.     Weeds  of  this  type  are  multi- 
plied rather  than  destroyed  by  plowing  and  discing. 

Underground  stems  may  be  much  elongated  or  they  may  be 
short  and  thick.     In  their  su})terranean  habit,   they  resemble 

roots,  and  one  may  easily 
mistake  some  types  of  them 
for  roots,  unless  the  stem 
characters  are  well  in  mind. 
However,  the  presence  of 
nodes,  internodes,  and 
leaves,  although  the  latter 
are  usually  scale-like,  serve 
to  identify  the  underground 
structure  bearing  them  as 
a  stem.  For  example,  the 
so-called  eyes  of  the  Irish 
Potato  are  buds  and  are  lo- 
cated in  the  axils  of  small 
scales  which  mark  the  nodes 
of  the  tuber.  (Fig.  155.) 
On  some  the  scale-like 


Fig.  15-i.  —  A  Woodbine  {Ampdopsis 
climbing  a  stone  wall,     a,  tendrils. 


Fig.  155.  —  Irish  Potato. 
e,  eyes;  s,  scale  leaves. 


leaves  are  large  and  fleshy,  while  on  others  they  are  very  incon- 
spicuous. Underground  stems  differ  so  much  that  they  have 
been  classified  into  rhizomes  or  rootstocks,  tubers,  bulbs,  and  corms. 
Rhizomes  are  very  much  elongated  underground  stems.  They 
are  so  named  because  of  their  resemblance  to  roots  (the  word 
rhizome  meaning  root-like).  They  are  commonly  called  root- 
stocks.    The  rhizome  is  one  of  the  most  common  forms  of  under- 


180 


STEMS 


ground  stems,  being  the  kind  of  underground  stem  most  common 
among  Grasses  and  weeds.  Many  wild  plants,  such  as  the  Ferns, 
May  Apple  or  Mandrake,  Solomon's  Seal,  Iris,  Water  Lily,  and 
others,  have  rhizomes.     {Fig.  156.) 


Fig.  156.  —  Rhizome  of  the  Mandrake  {Podophyllum),  showing  aerial 
shoot  and  two  scars  (o)  left  by  previous  shoots. 

One  striking  feature  is  the  difference  in  the  way  that  rhizomes 
and  their  aerial  shoots  respond  to  gravity  and  light.  While  their 
aerial  shoots  grow  toward  the  light  and  away  from  the  earth,  the 


UNDERGROUND  STEMS 


181 


rhizome  elongates  horizontally  under  the  surface  of  the  ground, 
neither  seeking  the  light  nor  growing  away  from  the  earth. 

Rhizomes  grow  best  at  certain  depths  in  the  soil,  and,  if  the 
depth  is  changed  by  adding  or  removing  soil  from  over  them,  they 
will  grow  up  or  down  until  the  required  depth  is  reached.  By  a 
covering  of  manure  or  straw,  the  rhizomes  of  Quack  Grass  and 
some  other  weeds  may  be  induced  to  grow  to  the  surface  or  even 
out  of  the  ground.  Such  weeds  are  sometimes  eradicated  by 
removing  the  covering  and  exposing  the  rhizomes  to  drying  and 
freezing  after  they  have  been  induced  to  grow 
to  the  surface. 

Rhizomes     elongate    and    push     forward 
through  the  soil  by  growth  at  one  end.     It 
is  near  this  growing  end  that  the  aerial  por- 
tions are  produced  from  season  to  season. 
As  the  rhizomes  push  forward,  the  older  por- 
tions behind  die  away,  and  if  the  rhizome  is 
branched,  as  many  of  them  are,  the  branches 
become    separated    and    form    independent 
rhizomes.    The  creeping  and  branching  habits 
of  rhizomes  are  important  features  for  vege- 
tative propagation.     Rhizomes  are  able  to 
creep  through  a  soil  which  is  already  well        ^    '        '       ^^^ 
occupied  by  other  plants,  and  consequently,   above  and  lengthwise 
plants  having  rhizomes  are  able  to  spread  section  below,   c,  main 
where  there  is  no  chance  for  seed  to  develop,  bud;  b,  small  buds;  s, 

The  tuber  occurs  among  plants  where  cer-  ^^^™'  '^'  roots;  /,  fleshy 
,    .  .  p    ,,  ]  ,      ,  .,      scales.    After  Andrews, 

tarn  regions  of  the  undergound  stem  or  its 

branches  become  much  enlarged  in  connection  with  food  storage. 

The  most  familiar  tuber  is  the  Irish  Potato.     The  nodes  are 

marked  by  the  scale-hke  leaves  in  the  axils  of  which  occur  the 

small  buds  or  eyes.     The  presence  of  nodes  identifies  the  Potato 

tuber  as  a  stem  structure.     It  is  the  stem  portion  of  tubers  that 

is  prominent,  the  leaves  and  buds  being  small.     Another  tuber 

with  nodes  more  prominent  than  in  the  Irish  Potato  and  also  of 

some  value  for  food  is  the  Jerusalem  Artichoke. 

In  hulhs  the  leaves  or  leaf  bases  are  more  prominent  than  the 

stem,  which  is  short,  erect,  and  enclosed  by  the  leaf  structures. 

Most  of  the  food  is  stored  in  the  leaf  structures  rather  than  in  the 

stem.     Some  common  bulbs  are  those  of  the  Onion,  Lily,  Hya- 


182 


STEMS 


cinth,  and  Tulip.  The  edible  portion  of  the  Onion  bulb  consists 
mainly  of  the  fleshy  scale-like  leaves,  in  which  the  food  has  been 
stored.     (Fig.  157.) 

Not  all  bulbs,  however,  are  produced  underground,  for  small 

Onion  bulbs,  called  hulh- 
lets,  often  replace  flower 
buds  in  the  common 
Onion.  These  small  bulbs 
are  sometimes  known  as 
"Onion  sets."  Some  Lilies 
also  produce  small  bulbs 
in  the  leaf  axils.  Such 
bulbs,  although  they  re- 
semble underground  bulbs 
in  structure,  are  not 
formed  in  connection  with 
underground  stems. 

Corms  are  very  short, 
solid,  vertical,  under- 
ground  stems,  usually 
bearing  roots  on  their 
lower  and  leaves  and  buds 
on  their  upper  surface. 
However,  buds  may  arise 
anywhere  and  roots  are 
sometimes  present  at  the 
upper  end  of  the  corm,  as 
in  the  Jack-in-the-pulpit. 
Corms  are  usually  marked 
by  scar-like  rings  left  by 
the  decay  of  former  leaves. 
From  the  buds  of  the  old 
corm  new  corms  develop.  {Fig.  158.)  The  most  familiar  corms 
are  those  of  the  Indian  Turnip,  Crocus,  Timothy,  Cyclamen,  and 
Gladiolus. 


Fig.  158.  —  A  corm  of  Gladiolus,  showing 
young  corms  developing  at  the  base  of  the 
old  one. 


General  Structure  of  Stems 

Stems  have  a  cylindrical  shape,  which  is  associated  with  the 
circular  arrangement  of  their  strengthening  tissue.  By  being 
arranged  in   a  circle   and  near  the  periphery  of  the  stem,  the 


GENERAL  STRUCTURE   OF   STEMS 


183 


strengthening  tissues  assume  a  tube-like  arrangement,  which  is  well 
known  to  engineers  as  the  arrangement  in  which  the  most  strength 
with  a  given  amount  of  material  can  be  secured.  The  truth  of 
this  principle  is  demonstrated  in  the  construction  of  bicycle 
frames,  where  much  strength  with  little  weight  is  secured  by  using 
large  tubes  instead  of  solid  rods.  Again,  having  the  cylindrical 
form,  stems  can  be  equally  resistant  to  strains  from  all  directions. 

Stems  taper  and  also  decrease  in  age  from  the  base  of  the  trunk 
to  the  end  of  the  twigs  where 
the  stem  tissues  are  in  process 
of  formation  from  the  apical 
meristems.  The  apical  meri- 
stems  are  also  known  as  pri- 
mary meristems  because  they 
form  other  meristems,  notably 
the  cambiums.  It  is  on  the 
new  elongation  at  the  tips  of 
the  stem,  that  the  leaves  appear 
anew  each  year.  The  nodes, 
the  regions  where  leaves  and 
buds  occur,  are  separated  by 
the  elongation  of  the  inter- 
nodes,  and  in  this  wa}^  the 
leaves,  which  are  younger  and 
more  crowded  the  nearer  the 
tip,  are  separated  and  exposed 
to  the  light.  In  most  annual 
stems  the  nodes  are  all  formed 
very  early,  and  elongation 
thereafter  consists  chiefly  in 
the  lengthening  of  the  internodes,  which  thereby  separate  the 
leaves  so  that  they  can  unfold  and  expand  to  their  mature  size. 
Thus,  as  shown  in  Figure  159,  the  nodes  and  internodes  of  a  Cora 
stem  are  all  present  in  a  Corn  seedling  two  or  three  weeks  old. 

In  most  herbaceous  stems,  where  there  is  no  need  of  corky  bark 
and  almost  the  entire  stem  is  leaf  bearing,  the  stem  is  active 
throughout  in  the  manufacture  of  food.  But  in  perennials,  such  as 
shrubs  and  trees,  photosynthesis  is  limited  to  the  young  branches 
where  the  leaves  are  borne  and  the  light  is  not  excluded  from  the 
green  cortex  of  the  branches  by  a  corky  covering.     In  passing 


Fig.  159.  —  Lengthwise  section 
through  the  stem  of  a  Corn  seedhng, 
showing  the  apical  meristems  (m),  the 
nodes  (n),  and  the  short  internodes  (i). 


184 


STEMS 


from  the  loaf  hearing  portion  of  a  branch  to  the  regions  behind 
where  food  manufacture  is  being  abandoned,  the  following  struc- 
tural features  are  plainly  seen. 

First,  there  are  the  leaf  scars,  left  where 
the  leaves  fell  away,  and  interesting  because 
of  the  way  they  are  formed.  {Ficj.  160.)  As 
the  time  approaches  for  leaves  to  fall,  a  cork- 
like layer,  known  as  the  absciss  layer,  forms 
across  the  base  of  the  leaf,  severing  the  direct 
connections  of  the  leaf  with  the  twig  and  re- 
maining as  St,  covering  over  the  scar  after  the 
leaf  falls.  The  absciss  layer  closes  the  open- 
ings which  would  otherwise  be  left  by  the 
falling  of  the  leaf,  and  thereby  prevents  the 
entrance  of  destructive  organisms  into  the 
twig.  It  is  in  connection  with  leaves  which 
still  remain  on  the  tree  after  the  absciss  layer 
is  formed  that  the  various  autumn  colors  oc- 
cur due  to  changes  in  the  dying  leaf  tissues. 

Second,    there   are   the  lens-shaped    dots, 
known  as  lenticels,  which,  although  common 
on  the  branches  of  all  woody  plants,  are  espe- 
cially   conspicuous    on 
Fig.  160.   Twig  of  the    branches    of    the 
the  Wliite  Walnut,    --^i  j     t>  ■      u 

,      .     ,    .         ,    '   Cherry   and    Birch. 

showins;  leai  scars  (a) .     ,  „ .  ,        

{Fig.  161.)  The  for- 
mation of  lenticels  accompanies  the  forma- 
tion of  bark.  In  the  young  twig,  where 
the  protective  covering  is  an  epidermis, 
air  is  supplied  to  the  tissues  beneath 
through  the  slit-like  openings  of  the  sto- 
mata;  but,  as  the  twig  becomes  older  and 
bark  is  formed,  the  stomata  are  replaced 
by  lenticels.  Lenticels  are  stomata  dis- 
torted and  transformed  in  structure  by  the 
development  of  bark.  Just  beneath  each 
stoma,  instead  of  cork,  there  is  formed  a 
loose  mass  of  cells,  and  this  loose  mass  of 
cells  is  pushed  up  into  the  opening  of  the  stoma,  as  shown  in 
Figure  162,  rupturing  the  stoma  and  surrounding  cells  and  thus 


Fig.  161.— Twig  of  Birch, 
showing  lenticels. 


GENERAL  STRUCTURE  OF  STEMS  185 

forming  a  lenticel.  Through  this  loose  mass  of  cells  air  easil}^ 
circulates  and  reaches  the  tissues  beneath.  As  the  twig  increases 
in  diameter  and  the  bark  is  stretched,  the  lenticels  are  enlarged 
and,  when  they  remain  visible  on  the  older  bark,  form  the  char- 
acteristic bands  as  on  the  older  bark  of  Cherries  and  Birches. 


Fig.  162.  —  Section  through  a  lonticel  of  the  Elder  {Sambucus  nigra),  e, 
epidermis;  ph,  cork  cambium  or  phellogen;  I,  loosely  joined  or  packing  cells 
of  the  lenticel;  pi,  cambium  of  the  lenticel.  Much  magnified.  After  Stras- 
burger. 

Third,  as  the  twig  becomes  older,  the  bark  increases  in  thick- 
ness, cutting  off  the  light  from  the  green  tissue  beneath,  which, 
consequently,  loses  its  green  color  and  no  longer  functions  in  the 
manufacture  of  food. 

On  each  leaf  scar  there  are  dots,  which  are  the  severed  ends  of 
the  vascular  bundles,  known  as  leaf  traces,  that  branched  off  from 
the  vascular  cylinder  of  the  stem  to  enter  the  leaf,  where  by  pro- 
fusely branching  they  form  the  veins  and  numerous  veinlets  of 
the  leaf.  In  turning  aside  to  enter  the  leaf,  the  leaf  traces  leave 
a  gap  in  the  vascular  cylinder  of  the  stem,  and  around  this  gap  the 
vascular  bundles  of  the  bud  in  the  axil  of  the  leaf  connect  with 
the  vascular  cylinder  of  the  stem.  {Fig.  163.)  Thus  through 
the  branching  and  rejoining  of  bundles  at  the  nodes,  a  plant's 
vascular  system  becomes  quite  complex,  looking  like  Figure  164- 

Stem  tips  are  not  covered  by  caps,  as  root  tips  are.  The 
actual  tips  of  stems  are  the  meristematic  tissues.     During  the 


186 


STEMS 


dormant  period,  primary  meristcms  are  usually  protected  by  bud 
scales,  while,  during  their  active  period,  they  receive  considerable 
protection  from  the  young  leaves,  which,  although  developing 
laterally  and  behind  the  tips, 
project  forward  and  are  usually 
so  crowded  and  folded  together 
that  they  hide  the  stem  tips. 

Behind  the  stem  tips  the  cells 
formed  from  the  primary  meri- 
stems  begin  to  elongate  and 
modify  into  tissues  and  con- 
tinue to  do  so  until  transformed 
into  the  mature  tissues  of  the 
older  parts  of  the  stem.  Stem 
tissues  differ:    (1)  in  some  im- 


FiG.  163.  —  Lengthwise  section 
through  the  apical  region  of  a  stem 
with  two  leaf  stalks  and  the  buds  in 
their  axils  included,  showing  the  con- 
nections of  the  vascular  bundles  of 
leaves  and  of  axillary  buds  or  branches 
with  the  vascular  cylinder  of  the  stem. 
The  vascular  cylinder  is  represented 
by  shaded  strands  on  each  side  of  the 
pith,  the  Ught  area  in  the  center  of 
the  stem.     Redrawn  from  Sargent. 


Fig.  IG-t.  —  Diagram  of  the  vascu- 
lar cylinder  of  the  young  stem  of 
Clematis  viticella,  showing  by  means 
of  dark  lines  the  branching  of  the 
vascular  bundles  at  the  nodes  to  sup- 
ply the  leaves  and  branches  with  bun- 
dles.    Modified  from  Nageli. 


portant  ways  according  to  whether  the  stem  is  monocotyledonous 
or  dicotyledonous;  and  (2)  in  minor  ways  according  to  whether 
the  stem  is  herbaceous  or  woody.  Thus  in  trees  the  tissues  of 
the  herbaceous  tips  differ  some  from  those  in  the  older  regions 
where  corky  bark  and  other  woody  features  are  developed. 


STRUCTURE  OF   MONOCOTYLEDONOUS   STEMS 


187 


Structure  of  Monocotyledonous  Stems 

The  most  useful  of  the  Monocotyledons  are  the  Grasses  of 
which  the  Bamboos  are  the  largest  representatives.  The  Lilies, 
Asparagus,  and  Palms  are  some  other  Monocotyledons  that  are 
familiar.  Nearly  all  Monocotyledons  are  herbaceous,  although 
there  are  a  few,  notably  the  Palms  and  Bamboos,  that  are  woody. 

The  characteristic  arrangement  of  the  tissues  of  monocotyle- 
donous stemS;  as  they  appear  to  the  naked  eye,  can  be  seen  in  the 


Fig.  165.  —  Cross  section  of  a  Corn  stem,     a,  rind;  v,  vascular 
bundles;   p,  pith;   c,  epidermis. 


cross  section  ol  a  Corn  stem,  as  shown  in  Figure  165.  In  this  sec- 
tion three  features  are  prominent.  First,  there  is  the  rind-like 
portion,  forming  the  outer  region  of  the  stem  and  affording  pro- 
tection and  strength.  The  cells  of  this  outer  region  contain 
chlorophyll  and  also  function  to  some  extent  like  leaves  in  the 
manufacture  of  food.  Second,  there  is  the  pith,  left  white  in  our 
drawing  and  filling  the  entire  cavity  within  the  rind.  Third,  there 
are  the  vascular  bundles  (shown  b}^  dots)  which,  although  scattered 
throughout  the  pith,  are  more  numerous  near  the  rind,  thus  tend- 
ing to  form  a  hollow  column,  which,  as  previously  pointed  out,  is 
the  best  arrangement  for  strength.  In  monocotyledonous  stems 
the  tissues  are  run  together  and  conseqm  ntly  are  not  so  grouped 


188 


STEMS 


as  to  form  distinct  regions,  such  as  pith,  vascular  cyUnder,  and 
cortex,  wliich  are  more  or  less  distinct  regions  in  Dicotyledons. 

When  cross  sections  of  the  Corn  stem  are  studied  with  the 
microscope,  such  anatomical  features  as  shown  in  Figure  166 
may  be  seen.  The  cells  of  the  rind  are  rectangular  in  shape,  con- 
sist of  a  number  of  rows,  and  their  walls  are  thickened  and  made 
woody  for  strength.  The  woody  feature  of  the  rind  is  character- 
istic of  Grasses  and  Sedges,  being  much  less  prominent  in  other 
monocotyledonous  stems,  as,  for  example,  in  Lilies  and  Aspara- 
gus. The  outer  row  of  cells  of 
the  rind  constitute  the  epider- 
mis, although  in  the  Grasses  the 
epidermal  cells  differ  very  little 
from  other  rind  cells,  except  that 
they  have  silica  and  cutin  de- 
posited in  their  outer  walls. 
The  vascular  bundles,  contain- 
ing numerous  cells,  show  three 
or  four  large  openings  which  are 
the  large  vessels  of  the  xylem. 
Besides  the  large  size  of  the  pith 
cells  as  shown  in  the  drawing, 
other  features  not  shown,  such 
as  their  storage  function  and 
their  being  so  loosely  joined  as 
to  form  a  spongy  filling  for  the 
stem,  should  be  mentioned. 
To  study  the  complex  structure  of  a  vascular  bundle,  we  must 
turn  to  a  more  highly  magnified  cross  section  of  the  bundle  as 
shown  in  Figure  167.  The  vascular  bundle  consists  of  strength- 
ening and  conductive  tissues,  the  latter  of  which  is  composed 
of  the  xylem  and  'phloem,  —  the  chief  structures  of  all  vascular 
bundles.  In  respect  to  the  character  of  the  vessels  composing 
them,  xylem  and  phloem  show  much  uniformity  throughout  Flow- 
ering Plants. 

In  the  xylem  the  conductive  tissues  consist  mainly  of  large  ves- 
sels, known  as  spiral,  annular,  or  pitted  vessels  according  to  the 
character  of  the  thickenings  in  their  walls,  as  partly  shown  in  Fig- 
ure 168  and  more  fully  shown  in  Figure  169.  The  woody  thicken- 
ings, which  strengthen  the  cellulose  walls  of  the  vessels  so  that 


Fig.  166.  —  A  portion  of  a  cross 
section  of  a  Corn  stem  much  en- 
larged, o,  epidermis;  6,  the  band  of 
strengthening  cells  under  the  epider- 
mis and  often  called  cortex;  v,  vascu- 
lar bundles;  e,  pith.    After  Stevens. 


STRUCTURE  OF   MONOCOTYLEDONOUS   .STEMS  189 

they  do  not  collapse  under  the  pressure  of  surrounding  tissues, 
may  form  rings  as  in  annular  vessels,  spirals  as  in  spiral  vessels, 
or  be  more  generally  distributed  over  the  wall,  leaving  only 
small  unthickened  areas  which  constitute  the  pits  characteristic 


Fig.  167.  —  Cross  section  of  a  vascular  bundle  of  Corn  highly  magnified, 
s,  strengthening  tissue;  j),  phloem  consisting  of  sieve  vessels  (e)  and  companion 
cells  (c);  X,  xylem  consisting  of  annular  vessel  (a),  spiral  vessel  (h)  and  pitted 
(t);  b,  parenchyma  cells. 


of  pitted  vessels.  The  xylem  vessels  are  free  from  protoplasm 
and  are  composed  of  cells  joined  in  series  with  end  walls  resorbed. 
They  are  known  as  tracheae,  and  are  quite  tube-like  in  struc- 
ture and  function.  Through  them  the  water  and  mineral  salts 
from  the  roots  are  carried,  some  reaching  the  leaves  and  buds 
while  much  leaks  out  through  the  cellulose  portions  of  the  walls 
to  supply  the  tissues  of  the  stem.  Around  the  vessels  are  the 
thin-walled  parenchyma  cells  which  may  function  some  in  con- 
duction. 

In  the  phloem  there  are  sieve  vessels  and  companion  cells.  The 
sieve  vessels  are  composed  of  cells  joined  in  series  and  so  named 
because  of  the  perforated  areas  occurring  in  their  end  and  side 


190 


STEMS 


Fig.  168.  —  Lengthwise  section  through  a  vascular  bundle  of  Corn,  the 
knife  sphtting  the  bundle  as  shown  by  the  hne  (o)  in  Figure  167,  thus  missing 
the  pitted  vessels,  x,  xylem  showing  spiral  vessel  (/i)  and  annular  vessels 
(a)  which  have  been  so  torn  by  the  growth  of  the  stem  that  only  the  rings 
are  left;  p,  phloem  consisting  of  sieve  vessels  (e)  and  companion  cells  (c); 
s,  strengthening  tissue.     Highly  magnified. 


FiQ.  169.  —  Vascular  elements  common  among  Ferns  and  Seed  Plants,  a, 
spiral  vessel;  h,  annular  vessel;  c,  pitted  vessel;  d,  reticulated  vessel;  e,  sca- 
lariform  vessel;  /,  elements  of  the  phloem  showing  sieve  vessel  with  sieve 
plate  (/*),  and  companion  cell  (c).  Highly  magnified,  a  and  h,  after  Bonnier 
and  Leclerc  Du  Sablon;  c,  after  De  Bary;  d,  modified  from  Barnes;  e,  modi- 
fied from  Cowles;   and/,  from  Strasburger. 


STRUCTURE   OF   MONOCOTYLEDONOUS   STEMS 


191 


walls.  The  sieve  vessels,  assisted  by  the  companion  cells,  which 
are  also  thin-walled,  elongated,  living  cells,  conduct  the  foods 
manufactured  in  the  leaves,  such  as  proteins  and  the  carbohy- 
drates of  which  sugar  is  the  chief  one.  The  strengthening  cells, 
which  are  more  numerous  at  the  outer  margin  of  the  xylem  and 
phloem,  form  a  sheath  around  the  vascular  bundle.  One  peculiar 
feature  of  the  vascular  bundles  of  Monocotyledons  is  that  there 
is  no  provision  whereby  the  bundle  can  increase  its  tissues,  and 
for  this  reason  it  is  known  as  a  closed  vascular  bundle.  In  mono- 
cotyledonous  stems,  where  there  is  no  special  provision  for  growth 


B 


Fig.  170.  —  Cross  sections  of  a  Barley  stem.  A,  section  across  the  en- 
tire stem  showing  the  hollow  (h)  and  the  outer  region  (o)  in  which  the  vascular 
bundles  occur.  B,  a  section  of  the  outer  region  much  enlarged,  r,  rind  com- 
of  strengthening  cells;  i',  vascular  bundles. 


in  diameter,  growth  is  mainly  in  length,  and  often  results  in  the 
development  of  extremely  slender  trunks,  like  those  of  Palms  and 
Bamboos. 

In  many  Grasses  the  stems  are  hollow  throughout  the  inter- 
nodes,  as  shown  in  Figure  170,  in  which  case  the  vascular 
bundles  are  limited  to  a  zone  just  within  the  rind.  In  most 
Monocotyledons  not  belonging  to  the  Grass  or  Sedge  family, 
the  outer  region  of  the  stem  is  less  firm  in  texture  and  in  a  few 
Monocotyledons,  as  in  the  Yuccas  and  Dragon  Tree,  some  of 
the  cells  in  the  outer  region  of  the  stem  divide  like  a  cambium, 
adding  cells  which  form  new  vascular  bundles  and  other  tissues. 
In  this  way  the  Dragon  Tree  may  continue  to  grow  in  diameter 
for  thousands  of  years  and  attain  a  diameter  of  many  feet. 


192 


STEMS 


Closed  vascular  bundles  and  their  scattered  arrangement  are 
the  chief  distinguishing  features  of  the  anatomy  of  monocotyle- 
donous  stems. 


f^ 


Structure  of  Herbaceous  Dicotyledonous  Stems 

Herbaceous  Dicotyledons  constitute  an  important  group,  for 
they  include  many  forage  plants,  notably  the  Clovers  and  Alfalfa, 
some  important  fiber  plants  as  Flax  and  Hemp,  most  vegetables, 


and  many 
are  a  few 


greenhouse  plants. 
Gymnosperms  that 


In  the  tropical  countries  there 

are  herbaceous,  but  in  general 

features    their    anatomy    is 

quite  similar  to  that  of 

herbaceous  Dicotyledons. 

All  stems  of  the  herba- 
ceous dicotyledonous  type, 
whether  they  are  stems 
strictly  herbaceous  through- 
out or  only  the  young 
branches  of  woody  stems, 
have  pith,  vascular  cylinder, 
and  cortex  which  occupy  well 
separated  regions  when  well 
developed.  Cross  sections 
appear  to  the  naked  eye 
about  as  shown  in  Figure 
171.  The  epidermis,  cortex, 
and  phloem  form  the  soft 
outer  zone,  while  the  xylem  forms  the  woody  cylinder  just  within 
the  soft  zone,  and  encloses  the  pith,  which  occupies  the  centei-  of 
the  stem.  In  order  to  trace  the  development  and  study  the 
anatomy  of  the  different  tissues,  we  must  turn  to  highly  mag- 
nified sections  as  shown  in  Figure  172. 

The  Cortex,  which  is  the  larger  part  of  the  outer  zone  of  tissues, 
is  covered  by  the  epidermis,  and  includes  the  starch  sheath  as  its 
innermost  layer.  Just  under  the  epidermis  some  of  the  cells  of 
the  cortex  are  transformed  into  collenchyma  cells,  which  are  par- 
ticularly abundant  in  the  angles  of  the  stem  shown  in  the  Figure 
but  more  generally  distributed  around  the  stem  in  many  other 
plants.  The  collenchyma  cells,  often  noticeable  in  sections  on 
account  of  their  whitish  glistening  appearance,  have  much  thick- 


FiG.  171.  —  Diagram  of  a  cross  section 
of  a  well  developed  herbaceous  stem,  show- 
ing the  epidermis  (a);  band  of  tissue  (b) 
composed  of  cortex  and  phloem;  xylem 
cylinder  (c) ;  and  pith  (r/) . 


STRUCTURE  OF  II1<]RBACK()US  DIC()T^'LJ<]IX)N()US  .STI':MS      193 


*  i  h 


A       f,    ^       V      s  a 


Fig.  172.  —  Diagrams  of  highly  magnified  sections  of  an  Alfalfa  stem. 
A,  both  cross  sectional  and  lengthwise  views  of  the  tissues  near  the  tip  of 
the  stem,  a,  epidermis;  I,  colhnichyina;  e,  chlorcnchyma  of  the  cortex;  s, 
starch  sheath;  i,  pericycle;  6,  bast  fibers;  /,  conductive  portion  of  the  phloem 
containing  the  sieve  tubes  and  companion  cells;  c,  cambium;  x,  xylem;  and 
p,  pith;  V,  vascular  bundle.  B,  section  farther  from  the  tip,  showing  cambium 
ring  and  the  closing  together  of  the  bundles.     Lettering  as  above. 


194 


STEMS 


ened  but  elastic  walls.  Being  fonued  early,  they  are  of  much 
importance  in  affording  strength  to  the  young  regions  of  the  stem 
where  bast  fibers  and  woody  tissues  are  not  yet  well  formed. 
Most  of  the  cortex  is  made  up  of  thin-walled  parenchyma  cells, 
known  as  chlorenchyma,  since  they  contain  chloroplasts  and  func- 
tion like  the  cells  of  leaves  in  the  manufacture  of  food,  being  sup- 
plied with  air  through  the  stomata  of  the  epidermis.  The  starch 
sheath,  comparable  to  the  endodermis  in  roots,  is  not  distinct 
from  the  other  cells  of  the  cortex  in  most  stems.     Its  function  is 


Fig.  173.  —  A  portion  of  a  cross  section  from  near  the  base  of  an  Alfalfa 
stem.  X,  xylem,  which  has  formed  a  compact  cylinder;  p,  pith;  c,  cam- 
bium;  t,  phloem;   e,  cortex;  a,  epidermis.     Highly  magnified. 


in  dispute.  Some  think  that  its  function  is  to  conduct  carbohy- 
drates, while  others  think  that  it  is  the  tissue  which  perceives 
geotropic  stimuli,  and  is  thus  responsible  for  the  direction  that 
stems  take  in  response  to  gravity. 

The  vascular  cylinder,  consisting  of  vascular  bundles  so  joined 
as  to  form  a  compact  cylinder  in  the  older  regions  of  the  stem,  as 
shown  in  Figure  173,  at  first  consists  of  separate  vascular  bundles 
having  a  circular  arrangement  about  the  stem  and  widely  sepa- 
rated by  bands  of  pith.  At  the  outer  border  of  each  mass  of 
phloem  are  hast  fibers,  often  called  sclerenchyma  fibers,  —  an  im- 


STRUCTURE  UF  HERBACEOUS  DICOTYLEDONOUS  STEMS     195 

portant  strengthening  tissue  common  to  all  dicotyledonous  stems. 
Centerward  and  matching  each  mass  of  phloem,  is  a  mass  of  xylem, 
wedge-shaped  in  outline  with  point  towards  the  center  of  the 
stem.  This  opposite  arrangement  of  phloem  and  xylem  contrasts 
with  the  arrangement  in  roots,  where  phloem  and  xylem  alternate. 
Between  the  phloem  and  xylem  is  the  cambium,  the  meristematic 
tissue  whereby  the  vascular  tissues  can  be  increased  indefinitely. 
Vascular  bundles  provided  with  cambium  are  called  open  bundles, 
and  are  characteristic  of  Dicotyledons  and  Gymnosperms, 
whether  herbaceous  or  woody.  During  further  development, 
the  cambiums  of  the  different  vascular  bundles  extend  through 


Fig.  174.  —  Lengthwise  section  through  a  vascular  bundle  of  a  herba- 
ceous dicotyledonous  stem,  x,  xylem  showing  pitted,  annular,  spiral  and 
scalariform  vessels;  p,  phloem  showing  sieve  vessels  and  companion  cells; 
c,  cambium.     Highly  magnified.     Modified  from  Hanson. 

the  intervening  pith  and  connect  to  form  the  continuous  cam- 
bium ring.  Then  due  to  the  activity  of  the  cambium  ring  in  the 
formation  of  other  vascular  bundles  between  those  first  formed 
and  in  the  enlargement  of  all,  the  intervening  pith,  excepting 
narrow  strands  of  it  called  pith  rays,  is  crowded  out,  and  finally 
a  compact  vascular  cylinder  as  shown  in  Figure  173  is  formed. 
In  many  herbaceous  Dicotyledons,  such  as  the  Giant  Ragweed 
an.d  others  that  grow  rapidly,  the  cambium  is  so  active  in  adding 
new  xylem  on  its  inner  side  and  new  phloem  on  its  outer  side 
that  both  phloem  and  xylem  constitute  zones  of  considerable 
thickness  at  the  end  of  one  summer's  growth.  The  zone  of 
xylem  is  often  so  prominent  that  the  basal  portions  of  such  stems 
are  considered  woody. 


196  STEMS 

The  vascular  bundles  of  all  Dicotyledons  are  very  sim- 
ilar to  those  of  Monocotyledons  in  structure  and  function 
of  conductive  vessels,  but  differ  essentially  in  having  cambium. 
(Fig.  174-)  The  conductive  tissue  of  the  xylem  consists 
chiefly  of  annular,  spiral,  pitted,  and  scalariform  vessels  —  the 
latter  being  so  named  because  the  thickened  areas,  separated 
by  slit-like  thin  areas,  are  so  arranged,  one  above  another,  as 
to  resemble  the  rounds  of  a  ladder.  As  in  Monocotyledons,  the 
xylem  vessels,  probably  assisted  by  the  neighboring  parenchyma 
cells,  are  the  passage  ways  through  which  the  water  and 
dissolved  substances  absorbed  by  the  roots  are  distriljuted 
throughout  the  shoot.     In  addition  to  sieve  tubes  and  companion 


Fig.  175.  —  Cross  section  of  a  Flax  stem,     a,  epidermis;  d,  bast  fibers; 
c,  cambimn;   p,  phloem;  x,  xylem,  h,  pith.     Enlarged. 

cells,  the  phloem  of  Dicotyledons  generally  contains  many  thin- 
walled  parenchyma  cells,  which  serve  in  conducting  the  carbohy- 
drates and  also  as  storage  places  for  proteins.  The  sieve  tubes 
and  companion  cells  conduct  the  proteins  and  a  part  of  the  carbo- 
hydrates. The  bast  fibers,  which  commonly  occur  in  connection 
with  the  phloem  of  all  Dicotyledons,  are  tough  flexible  strands 
adapted  to  afford  strength.  In  fiber  plants,  such  as  Flax  and 
Hemp,  the  bast  fibers  are  well  developed  and  their  importance 
in  the  manufacture  of  fabrics,  as  the  manufacture  of  linen  from 
Flax,  is  well  known.     (Fig.  175.) 

In  contrast  to  the  stems  of  Monocotyledons,  the  stems  of  Di- 
cotyledons and  Gymnosperms  have  as  their  distinctive  features 
the  circular  arrangement  of  vascular  bundles  and  the  presence 


STRUCTURE  OF  WOODY  STEMS 


197 


of  cambium.  The  stems  of  Dicotyledons  and  Gymnosperms, 
since  they  increase  in  diameter  by  the  addition  of  new  layers 
of  xylem  or  wood  on  the  outside  of  that  previously  formed, 
are  called  exogenous  stems.  The  stems  of  Monocotyledons  are 
called  endogenous  —  a  term  adopted  when  botanists  had  the  erro- 
neous notion  that  monocotyledonous  stems  grow  by  the  addition 
of  new  tissues  on  the  inside  of  the  older  ones. 

Structure  of  Woody  Stems 

Woody  stems,  characteristic  of  the  shrubs  and  trees  of 
Dicotyledons  and  Gymnosperms,  are  fundamentally  the  same 
in    structure    as     herbaceous    dicotyledonous    stems,    for    the 


Fig.  176.  —  Diagrammatic  drawing  of  a  cross  section  of  a  young  Apple 
twig,  e,  epidermis;  d,  cortex;  b,  bast  fibers;  p,  phloem;  c,  cambium;  i, 
xylem;   a,  pith. 

circular  arrangement  of  vascular  bundles  and  presence  of 
cambium  are  likewise  their  distinctive  structural  features. 
They,  too,  are  exogenous.  Their  herbaceous  tips,  being  similar 
in  structure  to  the  herbaceous  dicotyledonous  stems  just 
described,  need  no  special  attention.     (Fig.  176.)     Aside  from 


198 


STEMS 


the  lenticels  already  mentioned,  the  features  most  pecuhar  to 
woody  stems  are  the  annual  rings  of  the  woody  cyUnder  and  the 
corky  hark  which  replaces  the  epidermis  and  some  or  all  of  the 
cortex.  Also  the  medullary  rays  are  commonly  better  developed 
in  woody  stems  than  in  herbaceous  stems.  These  features  are 
directly  associated  with  the  perennial  habit  and  the  capacity  to 
add  new  layers  of  xylem  and  phloem  each  year  and  thus  increase 


Fig.  177.  —  Cross  section  of  an  Oak  branch  from  a  region  nine  years  old. 
0,  outer  corky  bark;  i,  inner  bark;  c,  cambium;  a,  aimual  rings;  m,  medul- 
lary rays;  p,  pith. 


in  diameter.  In  well  developed  woody  stems,  as  shown  in  Figure 
177,  there  are  three  regions,  bark,  woody  cylinder,  and  pith,  al- 
though the  latter  is  often  so  small  in  amount  as  to  appear  absent. 
The  bark,  consisting  of  outer  and  inner  bark,  the  latter  of  which 
contains  the  active  phloem,  extends  centerward  to  the  cambium, 
which,  although  distinctly  separating  the  bark  and  wood,  is  so 
inconspicuous,  except  under  the  microscope,  that  bark  and  wood 
appear  directly  joined.  The  annual  rings  are  the  circles  in  the 
wood,  and  the  medullary  rays  show  as  radiating  lines  travers- 


STRUCTURE  OF  WOODY   STEMS 


199 


ing  the  bark  and  wood,  reaching  part  way  or  all  the  way  to  the 
pith. 

The  bark,  characteristic  of  woody  plants,  is  originated  by  the 
cork  cambium  which  forms  as  an  inner  layer  of  the  epidermis 
or  in  the  cortex  beneath.  (Fig.  178.)  As  the  branch  increases 
in  diameter,  the  epidermis  seldom  grows  in  proportion,  but  usu- 


FiG.  178.  —  Diagrammatic  drawing  of  a  cross  section  of  an  apple  twig 
after  completing  two  seasons  growth,  c,  epidermis  sloughing  off;  k,  cork; 
h,  cork  cambium;  i,  inner  cortex;  n,  the  phloem  formed  the  first  season; 
7>,  phloem  formed  the  second  season;  c,  cambium;  x,  xylem  formed  the 
first  season;   y,  xylem  formed  the  second  season. 

ally  dies  and  sloughs  off,  and  its  protective  function  is  assumed  by 
the  cork  formed  beneath  and  gradually  thickened  as  the  stem 
grows  older.  In  some  cases  the  cork  cambium  produces  cortex 
cells  on  its  inner  side  as  well  as  cork  on  its  outer  side,  in  which 
case  the  cortex  is  increased  in  thickness.  Since  cork  is  imper- 
vious to  water,  the  tissues  on  its  outside,  having  their  water 
supply  cut  off,  soon  die  and  with  the  epidermis  and  cork  form 


200  STEMS 

the  dead  outer  bark.  In  a  few  trees  like  the  Beech  and  Fir  the 
original  cork  cambium  may  renew  its  activity  year  after  year,  but 
usually  the  cork  cambium  is  replaced  each  year  by  a  new  one 
formed  just  beneath.  The  inner  bark  consists  of  the  inner  cortex 
and  the  elements  of  the  phloem  made  up  of  sieve  tubes,  com- 
panion cells,  parenchyma  cells,  and  bast  fibers.  After  years  of 
growth  the  outer  layers  of  phloem  die  and  thus  on  trunks  of  trees 
of  much  age,  the  inner  living  bark  contains  only  the  inner  layers 


Fig.  179.—  Cross  section  throug;h  the  stem  of  a  Black  Oak,  showing 
heartwood  and  sapwood.     From  Pinchot,  U.  S.  Dept.  of  Agr. 

of  phloem,  the  older  layers  of  phloem  having  become  a  part  of  the 
outer  bark.  Due  to  the  addition  of  cork  and  the  increase  of  the 
phloem  and  woody  cylinder  in  thickness,  the  bark,  which  is  un- 
able to  increase  in  circumference  except  in  a  few  cases,  as  in 
Beeches,  is  usually  broken  and  slowly  exfoliated.  It  is  usually 
broken  into  furrows,  which  are  thought  to  serve  the  same  purpose 
as  lenticels  in  letting  air  into  the  stem  tissues  beneath. 

The  woody  cylinrler,  consisting  of  the  xylems  of  numerous  vas- 
cular bundles  closely  joined,  functions  chiefly  in  the  conduction 


STRUCTURE  OF  WOODY  STEMS 


201 


of  the  water  and  mineral  salts  supplied  by  the  roots.  However, 
much  stored  food  in  trees  is  transferred  to  the  growing  regions 
through  the  xylem.  This  is  especially  true  in  the  early  spring  as 
well  known  in  the  case  of  Maples  where  the  xylem  carries  a  large 
amount  of  sugar  in  the  spring  sap.  In  Gymnosperms  the  xylem 
consists  chiefly  of  tracheids  through  which  the  water  ascends  by 
passing  from  one  cell  to  another  through  the  thin  places  of  the 
bordered  pits  which  are  provided  in  their  cell  walls.     In  woody 


Fig.  180.  —  Piece  of  a  stem  of  Scotch  Fir  four  years  old,  showing  the 
medullary  rays  as  they  appear  in  cross,  radial,  longitudinal,  and  tangential 
longitudinal  sections  of  the  stem.     Enlarged  six  times.     After  Strasburger. 


Dicotyledons  the  xylem  consists  of  tracheae  of  the  annular, 
spiral,  pitted,  and  scalariform  type,  among  which  are  inter- 
mingled wood  fibers,  wood  parenchyma,  and  some  tracheids. 

The  xylem  tissues  formed  in  the  spring,  when  there  is  need  for 
rapid  transportation  of  water  and  dissolved  substances  to  the  ex- 
panding tips,  consist  of  large  cells  with  large  cavities  and  thus 
give  to  the  spring  wood  that  open,  porous  character  which  con- 
trasts so  much  with  the  compact  character  of  the  fall  wood  that 
the  annual  rings  result.  Annual  rings,  as  their  name  indicates, 
are  formed  usually  only  one  each  year  and  consequent!}'  their 
number  indicates  quite  well  the  age  of  the  tree.     Since  the  cam- 


202 


STEMS 


bium  adds  new  phloem  on  its  outside  at  the  same  time  that  it  adds 
new  xyleni  within,  annual  rings  occur  in  the  bark  as  well  as  in 
the  wood;  but  in  the  bark  where  the  tissues  are  soft  and  there- 
fore crushed,  the  annual  rings  are  either  indistinct  or  obliterated. 
In  some  woody  stems  having  many  annual  rings,  only  the  outer 
annual  rings  which  constitute  the  sap  wood,  recognizable  by  its 


Fig.  181.  —  A  piece  of  an  oak  board,  c,  end  of  the  board  showing  medul- 
lary rays  (m)  and  annual  rings  (?•) ;  a,  edge  of  the  board  showing  the  medul- 
lary rays  (m) ;  b,  broad  surface  of  the  board  showing  the  annual  rings  con- 
sisting of  light  and  dark  bands. 

light  color,  are  active  in  conducting.  (Fig.  179.)  Sap  wood  is 
often  called  the  living  wood  because,  although  much  of  it  is  dead, 
the  cells  of  the  medullary  rays  and  wood  parenchyma  are  alive, 
while  the  heart  wood  is  practically  all  dead.  Heart  wood  is  usu- 
ally recognized  by  its  dark  color  due  to  deposits  of  various  sub- 
stances, principally  in  the  cell  walls. 

The  medullary  rays  are  also  formed  by  the  cambium  and  are 
of  two  kinds:   (1)  those  extending  from  pith  into  bark  and  known 


STRUCTURE  OF  WOODY  STEMS  203 

as  primary  rays;  and  (2)  those  reaching  only  part  way  through 
the  wood  and  known  as  secondary  rays.  {Fig.  180.)  The  medul- 
lary rays  are  composed  of  thin-walled  living  cells,  which  function 
in  food  storage  and  in  the  transportation  of  materials  laterally 
through  wood  and  bark.  They  are  narrow  plates  of  cells,  one  or 
only  a  few  cells  in  thickness,  and  extend  up  and  down  through 
the  stem  only  very  short  distances  as  may  be  ascertained  in 
Figure  180. 

Both  annual  rings  and  medullary  rays  have  an  economic  im- 
portance in  connection  with  lumber,  where  they  form  the  beauti- 
ful figures  on  the  surface  of  cabinet  woods.  When  lumber  is 
quarter  sawed,  that  is,  sawed  so  that  the  broad  surface  of  the  board 
is  parallel  with  the  medullary  rays,  then  its  beauty  is  due  to  the 
medullary  rays  which  form  the  smooth-looking  blotches  as  shown 
on  the  edge  of  the  board  in  Figure  181.  When  plain  sawed,  that 
is,  sawed  at  right  angles  to  the  medullary  rays,  the  beauty  of  the 
board  is  due  to  the  figures  formed  by  the  annual  rings  as  shown 
on  the  broad  surface  of  the  board  in  Figure  181. 

In  summarizing,  corky  bark,  annual  rings,  and  prominent 
medullary  rays  may  be  stated  as  the  distinguishing  features  of 
woody  stems.  Like  herbaceous  dicotyledonous  stems,  they  are 
characterized  by  the  circular  arrangement  of  vascular  bundles 
and  presence  of  cambium  —  features  which  distinguish  them 
from  monocotyledonous  stems  where  the  vascular  bundles  have 
the  scattered  arrangement  and  cambium  is  absent. 


'yiyrj 


CHAPTER  X 


BUDS;    GROWTH   OF  STEMS;   PRUNING;    PROPA- 
GATION   BY   STEMS 

Buds 

Nature  of  Buds.  —  Buds  contain  a  partially  developed  portion 
of  a  stem  with  leaves  and  also  flowers,  when  present,  in  an  em- 
bryonic state.  A  close  study  of  buds,  like  those  of  fruit  trees, 
shows  that  the  stem  portion  contained  is 
very  short  and  that  the  leaves  and  flowers, 
although  they  may  be  seen  with  a  micro- 
scope of  low  power  or  often  with  the  naked 
eye,  are  very  rudimentary.  Buds  are  often 
defined  as  undeveloped  shoots.  The  most 
important  thing  about  a  bud  is  that  it  con- 
tains the  meristematic  tissues  upon  which 
growth  in  length  (primary  growth)  and  the 
formation  of  new  leaves  and  flowers  depend. 
For  this  reason,  when  the  bud  on  the  end  of 
a  branch  is  removed,  the  branch  can  grow  no 
more  in  length  at  that  point.     {Fig.  182.) 

Buds  are  common  to  all  plants,  but  they 
are  most  noticeable  in  perennials,  such  as 
Hickory  bud.  a,  furry  trees  which  have  dormant  periods  occurring 
inner  scales;  h,  outer  during  the  winter  season  in  temperate  re- 
scales;  Z,  folded  leaf;  m,  gions  or  during  dry  seasons  in  warm  coun- 
apical  meristem;  r,  re-  ^^-^^^  rj.^^  ^^^^  ^^  ^j^^^^  ^^^^^  ^^^  ^^^^^ 
gion  to  which  the  scales  .        ,      ,  ,  ^^  i      -ii 

are  attached.  Modi-  ^^  resting  huds  and  are  usually  covered  with 
fied  from  Andrews.  scales  which  protect  the  inner  portions  from 
drying  and  other  destructive  agencies.  The 
scales  overlap,  forming  a  covering  of  more  than  one  layer,  and 
are  often  made  more  protective  by  becoming  hairy  or  waxy.  Bud 
scales  are  closely  related  to  leaves  and,  in  most  cases,  are  simply 
modified  leaves.  Sometimes,  however,  they  are  modified  stipules 
which  are  leaf  appendages. 

204 


Fig.  182.  — Lengtl 
wise  section  through  a 


OPENING   OF   BUDS 


205 


In  plants,  like  annuals  and  those  that  live  in  the  tropics,  the 

buds  usually  have  no  protective  scales  and  are  called  naked  buds. 

Scaly  buds  are  characteristic  of  plants  which 

must  pass  through  seasons  that  are  unfavor- 
able for  growth,   and  may  be   considered  a 

device    for   maintaining    partially   developed 

stem  portions  in  a  protected   state,  and  in 

readiness    to    assume   rapid    growth    at    the 

opening  of  the  growing  season. 

Opening  of  Buds.  — 
The  bud  scales  are  forced 
open  by  the  growth  of 
the  young  shoot  within. 
The  resumption  of 
growth  by  the  parts  en- 
closed is  first  shown  by  ^lo.  183. -Flower 
the  swelling  of  the  bud.  bud  of  the  Pear,  in 
When  the  young  shoot  which  the  flowers  are 
resumes  growth  at  the  Pushing  the  scales 
beginning  of  the  grow-  ^P-t^nd  coming  out 
.  °    .  '^    .  ,     of  the  bud.     After 

ing  season,  it  grows  with  Bailey. 
remarkable  rapidity  and 
in  a  few  days  pushes  out  of  its  scaly  cover- 
ing. {Fig.  183.)  After  the  shoot  has  es- 
caped, the  scales  usually  fall  off,  leaving  a 
scar  about  the  branch  at  their  place  of  at- 
tachment. The  bud  has  now  disappeared 
and  in  its  place  there  is  a  new  growth  bear- 
ing leaves  or  flowers,  or  sometimes  both. 

The  scars  left  by  the  scales  remain  until 
the  bark  is  sufficiently  developed  to  obscure 
them,  and  serve  to  indicate  the  age  of  the 
different  regions  of  a  branch.  In  Figure 
184,  the  portion  beyond  the  scar  (a)  is  the 
last  season's  growth.  The  portion  between 
(a)  and  (b)  is  two  years  old,  and  the  por- 
tion between  (b)  and  (c)  is  three  years  of 
age.     Thus  the  age  of  a  given  region  of 

a  branch  is  indicated  by  the  scars  on  the  branch  as  well  as  by 

the  annual  rings  in  its  woody  cylinder. 


Fig.  18  4.  — Plum 
branch  showing  regions 
of  different  ages  as  in- 
dicated by  the  scars  re- 
sulting from  the  falling 
away  of  the  bud  scales. 
Described  in  text. 


206 


BUDS 


Position  of  Buds.  —  Buds  are  cither  terminal,  located  at  the 
tip  of  the  stein :  or  lateral,  occupying  positions  on  the  side  of  the 
stem.  {Fig.  185.)  The  phunulc  is  the  first  ter- 
minal bud  of  the  seedling.  The  terminal  bud  is 
usually  larger  and  stronger  than  the  lateral  ones, 
and  its  shoot  usually  makes  more  growth  than  the 
shoots  of  lateral  buds. 

Lateral  buds  usually  occur  in  the 
leaf  axils  and  when  so  located  are 
called  axillary  buds.  In  many  plants 
extra  buds  called  accessory  buds  oc- 
cur, which  may  stand  just  above  the 
axillary  bud,  as  in  the  Butternut,  or 
on  either  side  of  it,  as  in  the  Box-elder. 
{Fig.  186.) 

Buds,  called  adventitious  huds,  often 
spring  from  stems,  from  roots,  or 
even  from  leaves  with- 
out any  definite  order. 
In  propagation  by  cut- 
tings or  layers,  adven- 
titious buds  often  have 
an  important  part  in  the  formation 
of  roots,  and  sometimes  in  the  for- 
mation of  stems.  Thus  in  the  propa- 
gation of  Sweet  Potatoes,  adventitious 
buds  are  depended  upon  to  develop 
the  new  plants.  In  Figure  187  is 
shown  the  sprouts  springing  from  the 
adventitious  buds  on  the  stump  of 
the  Basket  Willow.  In  this  case  the 
sprouts  are  harvested  after  they  be- 
come large  enough  to  be  woven  into 
baskets,  and  a  new  lot  of  sprouts  is 
then  produced  from  other  adventitious 
buds.  In  this  way  one  can  secure 
many  crops  of  stems  from  one  stump. 
On  the  other  hand,  adventitious  buds  are  often  a  source  of 
trouble,  as  in  the  clearing  of  ground  where  the  sprouts  develop- 
ing from  the  adventitious  buds  on  the  stumps  and  roots  tend  to 


a- 


Fig.  185.— 
Branch  of  the 
Hickory,  show- 
ing large  ter- 
minal bud  (0 
and  smaller 
lateral  buds  (/). 


B 

■  Accessory  buds 
of  the  Butternut  and  Box-elder. 

A,  twig  of  Butternut;  t,  ter- 
minal bud;  a,  accessory  buds; 
X,   axillary  bud;    /,   leaf  scar. 

B,  accessory  buds  (a)  and  axil- 
lary bud  (x)  of  the  Box-elder. 
After  Bergen. 


WHAT   BUDS  CONTAIN 


207 


reoccupy  the  ground  from  which  the  trees  and  shrubs  have  been 
removed.  However,  in  case  of  some  valuable  trees  like  the 
Chestnut,  the  sprouting  habit  is  utilized  in  the  production  of 
a  new  crop  of  trees.  (Fig.  188.)  In  some  forage  plants,  as 
Alfalfa  illustrates,  a  number  of 
crops  of  hay  can  be  obtained 
each  year  because  of  the  contin- 
uous development  of  adventi- 
tious buds  on  the  crown  or  basal 
portion  of  the  stem.     {Fig.  189.) 

What  Buds  Contain.  —  Some 
buds  contain  only  flowers,  some 
only  leaves,  while  some  contain 
both  flowers  and  leaves.  Buds 
are  called  flower  buds,  leaf  buds, 
or  mixed  buds  according  to  what 
they  contain.  In  such  fruit  trees 
as  the  Apricot  and  Peach,  the 
buds  contain  only  flowers  or 
only  leaves,  while  in  the  Apple 
and  Pear  the  buds  contain  both 
flowers  and  leaves,  or  leaves 
only.     (Figs.  190  and  191.) 

Flower  buds,  or  fruit  buds  as 
they  are  often  called,  are  usually 
broader  and  more  rounded  than 
leaf  buds  and  can  often  be  iden- 
tified by  their  position  on  the 
branch.  For  example,  in  the 
Peach  and  often  in  the  Apricot 
the  fruit  buds  are  lateral  buds 
on  the  current  season's  growth, 
while  in  the  Apple  and  Pear  they 
are  usually  the  terminal  buds 
of  the  stunted  lateral  branches 
called  fruit  spurs  which  are  located  on  those  portions  of  the 
larger  branches  two  or  more  years  of  age.  In  Cherries  and  Plums 
the  fruit  buds  occur  in  clusters  on  the  sides  of  the  spurs.  In 
grapes  the  flowers  occur  on  the  sides  of  the  current  spring  shoots. 
The  shape  and  place  of  appearance  of  fruit  buds  varies  much  in 


Fig.  187.  —  Basket  Willow  from 
Avhich  many  crops  of  branches  are 
obtained  through  the  development 
of  adventitious  buds. 


208 


BUDS 


the  different  classes  of  fruits,  and  it  is  important  that  one  should 
know  their  location  and  time  of  formation,  for  such  information 
is  valuable  in  deciding  how  and  when  to  cultivate  and  prune. 

,  i         \      >     I 


Fig.  188.  —  Chestnut  sprouts  growing  from  stumps. 
After  Gifford  Pinchot. 

Formation  of  Buds.  —  The  buds  of  plants  which  have  a  rest 
period  are  formed  during  one  season,  lie  dormant  during  the  rest 
period,  and  open  at  the  beginning  of  the  next  growing  season. 
Thus  the  buds  of  our  fruit  trees,  which  produced  flowers  and 
leafy  shoots  this  year,  were  formed  last  year.  As  the  new  shoots 
develop  each  year,  new  buds  are  formed  in  the  axils  of  the  leaves 
and  at  the  apex  of  branches,  and  in  these  buds  are  the  flowers 
and  leaves  which  appear  the  following  year. 


FORMATION   OF    lUIDS 


209 


A  study  ^  of  the  development  of  the  buds  of  our  fruit  ti'ccs  iuis 
shown  that  the  parts  of  a  bud  are  formed  durhig  the  summer  and 
fall  and  are  often  so  well  developed  before  frost  comes  that  the 
flowers  and  leaves  mav  he  identified  if  sections  of  the  buds  are 


Fig.  189.  —  Alfalfa  plant,  showing  development  of  branches  on  the  crojvn. 
h,  a  main  branch  of  the  crown;  s,  stumps  of  branches  which  have  been  mowed 
off;  /(,  new  branches. 

studied  with  the  microscope.  Thus  the  character  or  content  of 
the  buds  of  our  fruit  trees  is  determined  several  months  before  the 
buds  open.  The  appearance  of  a  heavy  bloom  in  the  orchard 
means  that  the  conditions  prevailing  during  the  previous  summer 
and  fall  favored  the  formation  of  flower  buds.  It  is  common 
observation  that  fruit  trees  bloom  more  profusely  some  seasons 
than  others.  Evidently  there  are  certain  conditions  which  favor 
the  formation  of  flower  buds  and  by  controlling  these  conditions 
one  can  control  to  a  certain  extent  the  fruitfulncss  of  a  tree. 


1  Fruit-bud  Formation  and  Development. 
Virginia  Agr.  E.Kp.  Sta.,  1909-1910. 


Annual  Report,   pp.  159-205, 


210 


BUDS 


The  formation  of  flower  buds  is  known  to  be  closely  related  to 
the  food  supply.^  Flower  buds  arc  formed  in  greatest  abundance 
when  there  is  more  reserve  food  than  is  needed  for  growth.  When 
a  plant  is  growing  rapidly  tuid  using  all  the  food  as  fast  as  the 


Fig.  190.  —  Fruit  buds  of  the 
Apricot,  in  which  case  a  fruit  bud 
contains  a  single  flower  and  no 
leaves.     After  Bailey. 

leaves  make  it,  few  flower 
buds  are  formed.  Further- 
more, if  a  tree  has  exhausted 
its  food  supply  in  producing 
a  heavy  crop  of  fruit,  not 
many  flower  buds  are 
formed,  and  as  a  result  the 
tree  will  bear  very  little  fruit 
the  following  year.  Any  con- 
dition that  leads  to  an  ac- 
cumulation of  reserve  food, 
such  as  checking  growth  by 
the  removal  of  terminal  buds 
or  by  cutting  down  the  water 
supply  from  the  roots,  favors 
the  formation  of  flower  buds. 


Fig.  191.  — Twig  of  the  Crab  Apple 
■  at  time  of  blooming.  The  terminal  shoot 
(a)  has  developed  from  a  leaf  bud,  no 
flowers  being  produced,  while  the  lateral 
shoots  (b)  have  come  from  mixed  buds, 
both  leaves  and  flowers  having  been 
produced. 

Technical  Bulletifi  9,  New  Hampshire 


^  Studies  in  Fruit  Bud  Formation. 
College  Agr.  Exp.  Sta.,  1915. 

Some  Effects  of  Pruning,  Root  Pruning,  Ringing  and  Stripping  on  the  For- 
mation of  Fruit  Buds  on  Dwarf  Apple  Trees.  Technical  Bulletin  5,  Virginia 
Agr.  Exp.  Sta.,  1915. 


ACTIVE  AND   DORMANT   BUDS 


211 


Occasionally  ringing  is  cmploj^ed  to  induce  the  formation  of  fruit 
buds,  in  which  case  a  narrow  ring  of  bark  is  removed  from  the 
trunk  or  branches  in  order  to  sever  the  phloem,  and  thus,  by 
cutting  off  the  escape  of  the  foods  to  the  roots,  bring  a))()ut  their 
accumulation  in  the  branches.  Favorable 
conditions  for  food  formation  in  the  leaves, 
such  as  light  and  free  circulation  of  air 
and  the  addition  of  soil  fertilizers,  also 
have  an  effect  upon  the  formation  of 
fruit  buds. 

Active  and  Dormant  Buds.  —  Many 
more  buds  are  produced  than  can  develop 
into  branches,  for,  if  all  buds  were  to  de- 
velop, branches  would  be  so  numerous 
and  crowded  that  none  of  them  could  do 
well.  The  food  supply  and  proper  light 
relations  permit  the  expansion  of  only  a 
few  buds.  Consequently,  many  buds  lie 
dormant  one  or  more  seasons  or  through- 
out the  life  of  the  plant.  Usually  the 
more  terminally  located  a  bud  is,  the  more 
likely  it  is  to  be  active.  Thus  the  ter- 
minal buds  of  the  main  branches  are  less 
likely  to  be  dormant  than  the  terminal 
buds  of  the  branches  less  prominent,  and 
of  the  lateral  buds  often  a  large  per  cent 
remain  dormant.  An  examination  of  the 
Ijranches  of  most  trees  shows  many  leaf 
scars  with  dormant  buds  which  most  likely 
will  remain  dormant  and  finally  become 
obscured  by  the  thickening  of  the  bark, 
just  as  many  others  have. 

The  dormancy  of  buds  seems  to  be  due 
to  checks  imposed  upon  them  from  without  and  not  to  condi- 
tions within  the  bud,  for  most  dormant  buds  can  be  induced 
to  become  active  by  the  removal  of  the  active  buds.  Thus 
when  the  terminal  buds  of  branches  are  removed,  some  of  the 
dormant  lateral  buds  become  active.  Use  is  made  of  this  prin- 
ciple in  inducing  shade  trees  and  fruit  trees  to  acquire  certain 
desirable  shapes. 


Fig.  192.  —  Sweet 
Cherry,  a  type  of  tree  ia 
which  terminal  growth  is 
prominent,  resulting  in  the 
development  of  a  central 
shaft  called  a  leader. 
After  L.  H.  Bailey. 


212  BUDS 

In  most  cases  the  terminal  bud  of  the  main  branch  is  largest 
and  its  shoot  makes  the  most  growth  during  a  growing  season, 
sometimes  producing  a  growth  of  several  feet  in  a  season,  while 
growth  from  buds  not  so  terminall}^  located  is  usually  much 
less. 

The  shape  of  a  tree  depends  much  upon  the  relative  develop- 
ment of  main  and  lateral  branches.  When  terminal  growth  is 
very  strong,  lateral  growth  is  weak  and  the  tree  develops  a  cen- 


FiG.  193.  —  Sour  Cherry,  a  tree  which  has  strong  lateral  growth  and 
consequently  no  leaders.     After  L.  H.  Bailey. 

tral  stem,  called  leader,  with  lateral  branches  more  or  less  sup- 
pressed. This  kind  of  growth  is  common  among  Poplars,  Pines, 
and  even  some  fruit  trees  have  it,  as  the  Sweet  Cherry  in  Figure 
192  illustrates.  Trees  with  this  habit  of  growth  tend  to  grow  tall 
and  slender.  To  induce  such  trees  to  grow  low  and  bushy  the 
terminal  buds  must  be  removed,  so  that  lateral  branches  will  de- 
velop. When  terminal  growth  is  weak,  lateral  growth  is  stronger, 
and  the  tree  is  commonly  much  branched  and  leaders  are  absent, 
as  the  Sour  Cherry  in  Figure  193  illustrates.  This  habit  of 
growth  is  characteristic  of  Maples  and  many  other  trees.     There 


REGIONS  OF   GROWTH 


213 


are,  however,  some  plants,  like  the  Lilac  shown  in  Figure  194, 
in  which  the  terminal  bud  is  replaced  by  two  lateral  ones, 
but  such  is  not  the  rule  among  plants. 


Growth  of  Stems 

Phases  of  Growth.  —  In  growth  three 
things  occur:  (1)  the  addition  of  new 
cells  by  the  meristematic  tissues;  (2)  the 
elongation  of  cells;  and  (3)  their  modifi- 
cation into  tissues.  The  first  phase  must 
precede  the  other  two,  but  elongation 
and  modification  accompany  each  other, 
for  cells  begin  to  modify  into  tissues  be- 
fore completing  their  elongation.  In 
short-lived  plants,  such  as  annuals,  the 
first  phase  is  most  prominent  in  the  seed- 
ling stage,  during  which  most  of  the  cells 
upon  which  growth  in  length  depends  are 
formed  from  the  apical  meristems.  In 
Corn  most  of  the  cells  are  formed  during 
the  first  three  or  four  weeks  of  growth. 
During  the  remainder  of  the  growth  pe- 
riod the  cells  elongate  and  modify  into 
the  tissues  of  the  mature  stem.  In  per- 
ennials the  three  phases  are  repeated  the  Lilac,  showing  the  ter- 
each  year  as  is  well  illustrated  by  the  minal  buds  replaced  by  two 
yearly  growth  of  trees.  But  even  in  ^^*«^'^^  ""<^^- 
trees  most  of  the  cells  which  have  to  do  with  the  growth  in 
length  are  formed  in  the  buds  during  the  previous  year;  and  to 
their  remarkably  rapid  elongation  is  due  the  conspicuous  phase 
of  spring  growth  in  which  the  shoot  elongates  and  leaves  and 
flowers  expand  into  almost  full  size  in  a  few  days. 

Regions  of  Growth.  —  The  principal  regions  of  growth  are  at 
the  apices  of  stems,  where  growth  in  length  occurs  by  the  addi- 
tion of  new  nodes  and  internodes,  and  at  the  cambium  layer, 
where  growth  in  diameter  takes  place.  In  such  stems  as  those  of 
the  Grasses,  the  basal  portion  of  each  internode  functions  for  some 
time  as  a  meristem  and  thereby  aids  in  the  growth  in  length  of  the 
internode.     It  is  due  to  this  feature  that  Corn  stems,  before  they 


Fig.  194. —  Branch   of 


214 


GROWTH  OF  STEMS 


reach  maturity,  are  easily  broken  off  just  above  tlie  node.  Fur- 
thermore, in  having  this  meristematic 
zone,  stems  of  the  Grasses,  when  blown 
down,  are  able  to  become  partially  erect 
by  bending  in  the  region  of  the  node  due 
to  a  more  rapid  growth  of  this  region  on 
its  lower  side. 

Since  the  stem  segments  are  added 
in  succession  at  the  apex,  a  stem  soon 
comes  to  have  segments  in  various  stages 
of  development,  for  while  those  at  the 
apex  are  just  beginning  to  elongate,  those 
at  the  stem's  base  may  have  completed 
their  elongation  and  formation  of  tissues. 
This  feature  is  illustrated  in  Figure  195, 
although  none  of  the  segments  are  yet 
mature. 

Primary  and  Secondary  Growth.  —  In 
7  both   stems    and    roots,   apical   growth, 

since  from  it  the  tissues  of  the  stem  and 
root  first  originate,  is  called  primary 
growth,  while  growth  from  the  cambium 
is  known  as  secondary  growth  because  it 
is  chiefly  concerned  with  adding  more 
tissues  of  the  same  kind  to  those  already 
formed  from  the  apical  meristems.  Tis- 
sues are  also  called  primary  or  secondary 
b  MHIilll  \  according   to    whether    they  originated 

from  the  primary  meristems  or  from  the 
cambium. 

Character  and  Rate  of  Growth  in 
Stems.  —  Since  elongation  or  enlarge- 
ment is  the  most  conspicuous  phase  of 
growth,  it  is  employed  in  determining 
the  character  and  rate  of  growth.  Al- 
FiG.  195.  — Lengthwise  though  the  most  conspicuous,  neverthe- 
section  through  the  stem 

of  a  Corn  plant,  the  plant  being  about  two  feet  high.  I,  leaves;  t,  tassel, 
r,  region  of  stem  where  internodes  have  not  elongated,  a,  internodes  which 
have  undergone  the  most  elongation,  b,  meristematic  region  at  the  base  of 
the  internodes. 


CHARACTER  AND  RATE  OF  GROWTH  IN   STEMS       215 

less  it  is  so  slow,  except  in  a  few  cases,  that  it  is  imperceptible 
to  the  unaided  eye;  and,  therefore,  to  directly  observe  it,  the 
growing  organ  must  be  watched  under  the  microscope.  How- 
ever, in  measuring  growth  in  large  organs,  such  as  stems,  leaves, 
and  roots,  other  methods  that  are  more  convenient  arc  usually 
employed.     Thus  by  marking  a  stem  into  segments  as  shown 


Fig.  196.  —  Stem  of  a  seedling  marked  to  show  the  regions  of  most  elongation. 
A,  stem  just  after  marking.     B,  stem  after  a  few  hours  growth. 

in  Figure  196  and  observing  the  spread  of  the  marks  apart, 
one  can  easily  determine  what  part  of  the  stem  is  most  active 
in  elongation.  Special  kinds  of  apparatus  run  by  clockwork, 
one  of  which  is  known  as  the  auxograph  (meaning  "growth 
writer")  and  another  as  aiixa  no  meter  (meaning  "growth  meas- 
urer"), have  been  so  devised  that  the  rate  and  fluctuations 
in  growth  are  recorded  by  a  pen  which  indicates  tlie  character  of 
the  growth  throughout  a  considerable  period  by  curvatures  in  the 


216 


GROWTH   OF  STEMS 


line  which  it  makes  on  the  paper  carried  around  on  a  revolving 
drum.  {Fig.  197.)  Such  an  apparatus  has  the  advantage  in  that 
one  can  see  just  how  growth  proceeds  at  any  period  during  day  or 
night,  if  the  apparatus  is  so  manipulated  that  the  hour  at  which 
any  part  of  the  line  is  made  can  be  determined.  Measurements 
by  such  an  apparatus  show  that  the  rate  of  growth  of  an  organ 
is  not  uniform,  but,  beginning  slowly,  it  gradually  rises  to  a  point 
where  growth  is  most  rapid  and  then  gradually  falls  away,  finally 


Fig.  197.  —  Auxanometer  in  operation.  As  the  plant  elongates,  the  small 
pulley  {w)  revolves,  revolving  with  it  the  large  pulley  (r)  which  magnifies 
the  motion  and  transmits  it  to  the  marker  {z)  that  marks  on  the  drum  (/). 
The  drum  is  revolved  by  the  apparatus  {k)  at  its  base  and  this  apparatus 
is  connected  with  the  clock  (m).     After  Pfeffer. 


ceasing  as  the  organ  approaches  maturity.  This  mode  of  enlarg- 
ing, which  is  commonly  known  as  the  grand  period,  is  character- 
istic not  only  of  stems,  but  also  of  fruits,  flowers,  leaves,  and  roots. 
The  fundamental  cause  of  the  grand  period  in  any  organ  is  due  to 
the  fact  that  cells  themselves  enlarge  in  this  way.  Unlike  leaves, 
flowers,  and  fruit  where  the  expansion  is  quite  even  throughout, 
stems  expand  by  each  internode  going  through  its  grand  period 
independently  of  the  other  internodes.  Thus  between  the  upper 
internodes  in  which  the  grand  period  is  just  beginning  and  those 


CHARACTER  AND  RATE  OF   GROWTH   IN   STEMS       217 

toward  the  stein's  base  where  the  grand  period  is  over,  there  are 
internodes  in  various  stages  of  the  grand  period.  Due  to  the 
overlapping  of  the  grand  periods  of  the  different  internodes,  the 
elongation  of  the  stem  as  a  whole  is  quite  uniform.  In  roots 
the  grand  period  is  passed  through  ver}-  quickly  and  is  evident 
only  near  the  tip. 

The  rate  of  growth  also  depends  much  upon  the  kind  of  plant 
and  upon  moisture,  temperature,  and  light.  While  some  plants, 
like  Corn  and  Giant  Ragweeds,  grow  to  a  height  of  six  feet  or  more 
in  three  or  four  months,  the  seedlings  of  some  Pines  and  Oaks 
grow  only  a  few  inches  during  an  entire  season.  Some  vines  like 
the  Hop  plant  may  grow  a  stem  more  than  twenty-five  feet  in 
length  in  one  growing  season.  Most  weeds  grow  more  rapidly 
than  cultivated  plants  and,  if  let  alone,  soon  exceed  them  and  cut 
off  the  light.  Measurements  have  shown  that  some  kinds  of 
Beans  and  Peas  can  elongate  about  two  inches  and  Wheat  about 
four  inches  in  forty-eight  hours.  In  perennials,  such  as  trees, 
growth  is  very  rapid  in  the  spring,  after  which  it  slows  down  dur- 
ing the  remainder  of  the  season. 

The  moisture  of  the  soil  and  air  is  an  important  factor  in  growth. 
It  is  common  knowledge  that  plants  are  checked  in  growth  when 
the  ground  becomes  dry.  The  moisture  of  the  air,  although  not 
of  use  to  the  plant  in  the  same  way  that  the  soil  moisture  is. 
checks  the  evaporation  from  the  plant  and  thereby  influences 
growth.  When  the  atmosphere  is  full  of  water,  as  on  "muggy" 
days,  there  is  not  much  evaporation  and  the  cells  easily  retain  the 
high  turgor  pressure  upon  which  rapid  growth  depends.  It  is 
partly  due  to  the  greater  humidity  at  night  that  many  plants 
grow  faster  then  than  in  the  day  time.  That  the  cells  of  plants 
are  often  more  turgid  at  night  than  in  the  day  time  is  shown  by  the 
fact  that  soft  stems,  like  those  of  Corn  and  Sorghum,  are  more 
flexiljle  and  not  so  easily  broken  off  in  the  latter  part  of  the  day  as 
they  are  at  night  or  in  the  morning.  For  this  reason,  the  after- 
noon, when  the  cells  are  least  turgid,  is  the  best  time  to  lay-b}^ 
Corn.  The  function  of  water  in  enabling  cells  to  stretch  is  an 
important  one,  for  enlargement  consists  in  stretching  the  proto- 
plasm and  cell  walls  without  much  increase  at  first  in  dry  weight. 
Thus  the  dry  weight  of  an  internode  of  a  stem  is  about  the  same 
at  the  end  as  at  the  beginning  of  the  grand  period,  although  the 
size  may  increase  many  times.     In  fact  seedlings,  before  they 


218 


GROWTH  OF  STEMS 


become  active  in  the  manufacture  of  food,  often  have  a  dry 
weight  less  than  that  of  the  seed. 

Temperature  is  the  most  important  factor  in  growth  and,  just 
as  in  the  germination  of  seeds,  the  minimum,  maximum,  and 


Fig.  198.  —  Two  Potato  plants,  one  of  which  was  grown  in  the  dark  and 
the  other  in  the  hght.  A,  plant  grown  in  the  dark.  B,  ])lant  grown  in  the 
Ught.     After  Pfeffer. 

optimum  temperature  for  growth  vary  with  the  kind  of  plant 
as  shown  in  the  table  on  next  page.  For  example,  the  optimum 
temperature  is  between  90°  and  95°  for  Corn,  between  80°  and 
85°  for  Barley,  and  about  70°  for  White  Mustard,  one  of  the 
weeds.     Thus  when  the  days  and  nights  are  so  cool  that  such 


CHARACTER   AND   RATE   OF   GROWTH   IN   STEMS       219 

plants  as  Corn,  Beans,  and  Pumpkins  grow  slowly,  White  Mus- 
tard and  other  plants  with  a  low  optimum  grow  rapidly.  In  gen- 
eral, arctic  plants  have  a  lower  optimum  than  tropical  plants,  and 
consequently  plants  transferred  from  one  region  to   the  other 


Fig.  199.  —  Pines  growTi  much  crowded  and  consequently  producing  slender 
trunks.    From  Bulletin  24,  North  Carolina  Geological  and  Economic  Survey. 

usually  do  not  thrive  until  they  become  acclimated,  that  is, 
until  the  plant's  protoplasm  becomes  adjusted  to  the  tempera- 
ture of  the  region. 


CJROWTH   TEMPERATURES   IN   FAHRENHEIT 


Opti 


Barley 

White  Mustard 

Scarlet  Runner  Bean 

Corn 

Pumpkin 


Deg. 

41 

32 

49-50 

49-50 

56-57 


Deg. 

83-84 

69-70 

92-93 

92-93 

92-93 


Deg. 
99-100 
82-83 
115-116 
115-116 
115-116 


220  GROWTH  OF   STEMS 

Indirectly  light  is  very  essential  for  growth  because  of  its  im- 
portance in  the  manufacture  of  plant  foods.  But  directly  light 
has  little  effect,  unless  it  is  intense,  and  then  it  checks  growth. 
That  most  plants  grow  faster  at  night  than  in  day  time  is  well 
known;  and,  although  much  of  the  increase  in  the  rate  of  growth 
at  night  is  due  to  the  greater  humidity  of  the  air,  some  is  due  to 
the  absence  of  the  inhibitive  effect  that  the  sun's  rays  have  on 


Fig.  200.  —  Pines  growing  in  the  open  where  their  trunks  are  short  and 
much  branched.  From  Bulletin  S4,  North  Carolina  Geological  and  Economic 
Survey. 

growth.  In  Bacteria,  where  the  protoplasm  is  not  protected  by 
pigments,  the  sun's  rays  so  inhibit  growth  that  they  have  an  im- 
portant germicidal  effect. 

On  the  other  hand,  if  plants  do  not  have  sufficient  light,  they 
Eire  affected  in  various  ways.  For  example,  when  plants  are  grown 
in  the  dark,  as  the  Potatoes  in  Figure  198  illustrate,  the  stems  are 
excessively  elongated,  the  leaves  are  abnormal,  and  the  plant 
lacks  chlorophyll,  on  which  account  the  plant  is  said  to  be  etio- 
lated. Even  plants  grown  in  the  shade,  having  the  light  only  par- 
tially cut  off,  are  usually  taller  and  more  slender  than  plants 


PRUNING  221 

grown  in  the  light.  Thus  man}-  forest  trees  which  hav'e  short, 
thick,  and  much  branched  trunks,  when  growing  in  pastures,  grow 
tall  slender  stems  with  branches  only  at  their  tops  when  grown 
in  forests  where  they  are  much  shaded.  It  is  for  this  reason  that 
most  forest  trees  grow  trunks  more  valual^le  for  lumber  when 
grown  in  thick  stands.  (Figs.  199  and  200.)  This  principle  is 
observed  in  growing  Sorghum  and  Corn  chiefly  for  fodder,  in  which 
case  the  plants  are  grown  in  thick  stands,  so  that  their  stems  will 
be  finer  and,  therefore,  better  for  feed.  Such  a  response  to  shade 
is  often  an  advantage  to  plants,  for  it  is  through  the  elongation 
of  their  stems  that  plants  compete  for  light  by  endeavoring  to 
raise  their  leaves  above  the  shade  of  neighboring  plants. 

Also  the  development  of  stem  tissues  is  more  or  less  influenced 
by  light.  Stems  grown  in  diminished  light  do  not  have  their 
mechanical  tissues  so  well  developed.  For  example,  when  grain 
plants  receive  insufficient  light  on  account  of  being  much  crowded, 
they  have  commonly  weak  stems  and  are  likely  to  lodge.  The 
bast  fibers  of  flax  are  finer  when  the  plants  are  thick  on  the 
ground,  and  when  flax  is  grown  for  fibers  it  is  commonly  grown 
in  thick  stands. 

Pruning 

Pruning  consists  in  cutting  away  portions  of  the  plant  and  is 
done  for  reasons  too  numerous  for  more  than  a  few  to  be  men- 
tioned here. 

First,  trees  that  tend  to  grow  tall  and  slender  may  be  induced 
to  acquire  a  low  thick  top  by  subjecting  them  to  the  process  called 
"heading-in,"  which  consists  in  pruning  the  main  branches  so  that 
growth  in  height  is  checked  and  a  good  development  of  lateral 
branches  is  induced.  This  method  is  often  used  in  controlling 
the  shape  of  shade  and  fruit  trees.  It  is  by  this  means  that  hedges 
are  made  to  grow  low  and  dense  and  thus  capable  of  turning  stock 
when  used  for  fences. 

Second,  often,  as  in  case  of  fruit  trees,  pruning  has  for  its  pur-' 
pose  the  checking  of  growth  which  has  been  so  thoroughly  ex- 
hausting the  food  supply  as  to  result  in  a  shortage  of  fruit  buds. 
In  this  case  growth  is  checked  by  removing  the  terminal  buds 
from  the  leaders  and  the  food  supply  thereby  conserved. 

Third,  plants  are  sometimes  pruned  to  delay  maturity.  For 
example,  in  growing  Sweet  Peas  the  young  pods  are  pinched  off 


222 


PRUNING 


so  as  to  conserve  the  food  material  and  thereby  prolong  the  flower- 
ing period  of  the  plant.  In  contrast  to  this  practice,  often  in 
case  of  nursery  trees,  the  leaves  are  stripped  off  so  as  to  cut  off 
the  food  supply  and  thereby  hasten  maturity  in  order  that  the 
trees  may  be  in  a  better  condition  to  stand  the  winter. 

Fourth,  fruit  trees  are  often  pruned  to  induce  the  development 
of  an  open  head  so  as  to  secure  better  lighting  for  the  interior 
branches.  Such  pruning  is  necessary  in  trees  with  heads  so  com- 
pact that  the  interior  branches  are  not  able  to  function  properly 
in  the  manufacture  of  food  or  in  bearing  fruit  because  of  the  lack 
of  light. 

Fifth,  when  fruit  trees  are  set  out,  it  is  necessary  to  prune  the 
top  to  safeguard  the  trees  against  injuries  from  excessive  evapo- 


Ni; 


^P 


m 


B 


Fig.  201.  —  A,  tree  just  received  from  nursery.  B,  same  tree  with  top 
and  roots  pruned  in  preparation  for  setting  in  the  ground.  From  Alfred 
Gaskill. 


ration.  Since  the  trees  have  their  absorbing  power  much  reduced 
through  the  loss  of  many  roots  broken  and  cut  ,away  in  trans- 
planting, the  development  of  a  large  leaf  surface  must  be  prevented 
or  the  intake  at  the  roots  and  outgo  at  the  leaves  will  not  be  prop- 
erly balanced.     (Fig.  201.) 

Sixth,  the  appearance  of  a  tree  as  well  as  its  protection  against 
further  injury  requires  the  removal  of  dead  and  diseased  branches. 
One  can  do  much  toward  preventing  some  plant  diseases,  such 
as  Fire  Blight  and  Black  Knot,  from  spreading  to  healthy 
trees  by  removing  and  burning  the  diseased  branches  of  affected 
trees. 


PRUNING 


223 


Seventh,  by  a  severe  pruning  of  the  top,  trees  which  are  beginning 
to  fail  from  general  debility  are  often  rejuvenated.  This  kind  of 
pruning,  which  is  characterized  as  severe  because  so  much  of  the 
top  is  removed,  is  known  as  "pruning  for  wood."     By  the  removal 


'JDJ  Vii   \i)pl(>  troo  whuh  has  Ix'oii  .sovou^lv  pruned,  its  main 

l)ranchos  Ihivnig  been  cut  back.     After  G    11   Po^\ell 


of  much  of  the  top  the  balance  between  the  top  and  roots 
is  upset,  and  as  a  result  a  much  larger  supply  of  water  and 
mineral  salts  is  received  by  the  remaining  branches,  which  con- 
sequently become  invigorated  and  mucl\  more  active  in  growth. 
(Fig.  202.) 


224 


PRUNING 


Fig.  203.  — Twigs 
showing  the  cuts  at  different  dis- 
tances from  the  bud.  A,  the  cut 
is  too  far  from  the  bud.  B,  the 
cut  is  so  near  the  bud  that  the 
bud  is  probably  injured.     C,  the 


Wounds  and  their  Healing.  —  The 
removal  of  a  branch  exposes  the  stem 
tissues,  and  makes  an  opening  where 
destructive  organisms,  which  may 
injure  or  even  destroy  the  plant,  can 
enter.  Unless  wounds  are  quickly 
healed  over,  the  plant  will  suffer. 

Since  tissues  that  are  much  spe- 
ciaUzed,  such  as  wood  and  corky 
bark,  have  lost  their  ability  to  grow, 
the  meristematic  tissues  or  cambiums 
must  be  depended  upon  to  heal  the 
wound.  If  the  conditions  are  favor- 
able for  growth,  the  cambiums  and 
the  cells  newly  formed  from  them 


cut  is  at  the  proper  distance  from   develop  a  mass  of  tissue  known  as 

obli'^uel  ?^^^^''''*^'''"*''^'''^''  ^^^  '^^^^''^'  "^^'"^^  spreads  over  the 
wound  and  forms  a  cap-like  covering. 
The  development  of  the  callus  depends  very  much  upon  the 
nature  of  the  wound  and 
where  it  is  made.  The  cut 
should  be  made  with  a  sharp 
tool,  and  so  made  that  the 
stem  will  not  be  split.  When 
a  small  branch  is  cut  off,  the 
cut  should  be  made  just  above 
a  bud,  as  shown  in  Figure  203, 
so  that  the  leaves  developed 
from  this  bud  will  supply  food 
for  the  formation  of  the  callus. 
If  the  wound  is  too  far  above 
a  bud,  or  if  the  cut  is  so  close 
that  the  bud  is  destroyed, 
then  there  will  be  a  dead 
stump  which  will  not  heal. 
Side  branches  should  be  Fig.  204.  —  An  example  of  bad  prun- 
pruned  close  to  the  main  ing,  showing  the  dead  stubs  of  branches 
u  u  au   i  i-u  1  •  which  may  lead  to  the  destruction  of 

branch,  so  that  the  cambmm  ^^e  tree.    After  Bailey, 
of  the  main  branch  ca©  heal 
the  wound.     In  Figure  204  is  shown  an  example  of  improper 


CUTTINGS 


225 


Fig.  205.  —  An  example  of  a  wound 
so  made  that  a  callus  is  closing  over  it. 
After  Bailey. 


pruning,  in  which  case  there  is  a  stump  which  wiU  not  heal  and  its 
decay  may  result  in  the  destruction  of  the  tree.  In  Figure  205 
is  shown  a  cut  made  in  the 
proper  way.  In  this  case  a 
callus  is  forming  and  enclos- 
ing the  wound.  i^      y^*!  ' 

Pl 

The  Propagation  ^   of  Plants 
by  Means  of  Stems 

Some  plants,  of  which  the 
Irish  Potato  is  a  familiar  ex- 
ample, are  propagated  ahnost 
entirely  by  planting  portions 
of  their  stems,  which  are  ca- 
pable of  developing  roots  and 
shoots  from  their  nodes. 
{Fig.  206.)  A  notable  exam- 
ple in  Southern  countries  is 
the  Sugar  Cane,  which  is  propagated  by  planting  sections  of 
stalks  from  which  new  plants  develop.  In  the  propagation  of 
fruit  trees,  Grapes,  Cranberries,  Roses,  Geraniums,  Carnations, 
and  many  other  plants,  stems  are  used,  although  not  always  in 
the  same  way. 

Propagation  by  stems  is  often  preferable  to  propagation  by 
seeds,  because  by  the  former  method  the  new  plants  are  more 
likely  to  be  of  the  parent  type.  This  fact  is  demonstrated  in 
propagating  Apple  trees,  which  seldom  come  true  from  seeds,  but 
do  when  propagated  by  grafting.  Another  advantage  of  propa- 
gation by  stems  is  that  new  plants  can  be  obtained  in  less  time 
than  by  seeds.  By  means  of  cuttings  new  Geraniums  or  Carna- 
tions of  considerable  size  are  obtained  in  a  few  weeks.  Propa- 
gation by  stems  may  be  by  cuttings,  layering,  grafting,  or  hudding. 

Cuttings.  — ■  In  the  study  of  prostrate  and  underground  stems, 
it  was  noted  that  nodes  of  stems  can  develop  roots  as  well  as 
shoots.  This  makes  it  possible  for  a  portion  of  a  stem  to  become 
an  independent  plant  under  proper  conditions.  Consequently, 
many  plants  are  reproduced  by  setting  detached  portions  of  their 

1  The  propagation  of  plants.  Farmers'  Bulletin  157,  U.  S.  Dept.  of  Agri- 
culture. 


22G      THE   PROPAGATION   OF   PLANTS   BY   MEANS   OF   STEMS 

steins  in  soil,  sand,  or  water  where  they  develop  roots  and  become 
as  self-supportins  as  the  parent  plant.  Such  detached  portions 
are  known  as  cuttings  and  consist  of  a  small  portion  of  a  stem,  as 
Figure  207  illustrates,  or  only  of  a  leaf,  as  in  the  propagation  of 


Fig.  207.  —  Geranium  cut- 
Fk;.  206.  —  The  Irish  Potato,  showing  new        ting,  showing  the  roots  devel- 
phmts  developing  from  the  eyes.  oping  at  the  cut  end. 

Begonias  and  a  few  other  plants  having  fleshy  leaves  as  shown  in 
Figure  208.  Among  cultivated  herbaceous  plants  which  are 
propagated  by  cuttings,  the  Irish  Potato,  Geranium,  Carnation, 
and  Coleus  are  familiar  examples.  In  Southern  countries  the  use 
of  cuttings  is  well  illustrated  in  the  propagation  of  Sugar  Cane, 
as  shown  in  Figures  209  and  210.  Other  plants  of  the  Grass  fam- 
ily, as  Johnson  Grass  and  Bermuda  Grass,  are  sometimes  propa- 
gated by  cutting  the  underground  stems  into  short  pieces,  which 
are  used  in  setting  fields  to  grass.     Unintentionally,  but  often  to 


GRAFTING 


227 


his  sorrow,  the  fanner  helps  bad  weeds,  such  as  Quack  Grass  and 
Marsh  Smartweed  {Polygonum  Muhlenbergii),  to  spread  by  scat- 
tering portions  of  their  underground  stems  while  putting  in  and 
cultivating  crops. 

Cuttings,  known  as  hard-wood  cuttings,  are  commonly  employed 
in  propagating  such  woody  plants  as  the  Grape,  Currant,  Goose- 
berry, Willows,  Poplars, 
and  many  ornamental 
shrubs.  They  may  be 
made  in  different  ways  as 
shown  in  Figure  211,  but 
in  each  case  they  must 
have  at  least  one  bud. 

Layering.  —  A  layer  is 
a  branch  which  is  put  in 
contact  with  the  soil  and 
induced  to  develop  roots 
and  branches  while  still 
in  contact  with  the  parent 
plant.  After  a  layer  has 
developed  roots  and 
branches,  it  is  separated 
from  the  parent  and  be- 
comes an   independent 

plant.  There  are  different  methods  of  layering,  but  usually  the 
branches  are  bent  to  the  ground  and  covered  with  dirt.  In 
layering  Grapes,  a  vine  is  stretched  along  in  a  shallow  trench 
and  buried  throughout  its  entire  length  as  shown  in  Figure  212. 
Raspberries  and  many  shrubs  are  propagated  by  layering. 

Grafting.  —  Grafting  is  the  common  method  used  in  propa- 
gating fruit  trees,  and  consists  in  so  joining  parts  of  different 
plants  that  they  unite  their  tissues  and  live  together  as  one  plant. 
In  grafting  there  are  two  members  involved,  the  stock  and  don 
or  scion.  The  stock,  which  may  be  a  root,  stump,  or  almost  the 
entire  shoot,  is  the  member  which  remains  in  contact  with  the  soil, 
while  the  cion  is  the  portion  of  a  shoot,  usually  a  twig  or  branch, 
which  is  to  be  made  to  grow  on  the  stock.  Since  only  growing 
tissues,  such  as  the  cambiums,  are  able  to  unite  and  heal  wounds, 
it  is  necessary  in  grafting  to  have  the  cambiums  of  the  stock  and 
cion  so  adjusted  that  they  can  become  grown  together  and  thus 


Fig.  208.  —  The  Life  Plant  {Bryophyllum 
calycinum)  developing  young  plants  on  the 
margins  of  the  leaves.  About  one-half  natu- 
ral size. 


228      THE   PROPAGATION   OF   PLANTS  BY   MEANS  OF  STEMS 

form  a  perfect  union.  {Fig.  213.)  However,  only  plants  closely 
related  can  be  successfully  grafted,  for  in  the  protoplasms  of  un- 
related plants  there  are  factors,  probably  differences  in  chemical 
nature,  which  prevent  the  union  of  the  cambiums. 


Fig.  209.  —  Cuttings  of  Sugar  Cane.  A,  cutting,  showing  two  nodes  and 
a  bud  at  each  node.  B,  cutting,  showing  a  new  plant  which  has  developed 
from  a  bud  at  the  node.     Adapted  from  N.  A.  Cobb. 

When  grafting  is  successful,  the  cion  becomes  as  closely  related 
to  the  activities  of  the  stock  as  ordinary  branches  are.  Through 
the  stock  the  cion  receives  water  and  mineral  elements  from  the 
soil,  while  the  stock  receives  some  of  the  foods  made  by  the  leaves 
of  the  cion.  However,  with  all  of  this  close  connection,  the  nature 
of  both  stock  and  cion  remains  in  most  cases  practically  unchanged 
and  each,  therefore,  continues  to  produce  fruit  unchanged  in  type. 
This  feature  is  important  for  two  reasons.  First,  it  enables  one 
to  combine  the  desirable  features  of  two  plants  into  one  individual 
where  the  desirable  features,  although  remaining  unchanged  in 
nature,  may  assist  each  other  in  functioning.  Some  fruit  trees 
bear  delicious  fruit,  but  on  account  of  poor  root  systems  or  other 


GRAFTING  229 

causes  they  are  not  hardy.  On  the  other  hand,  some  trees  are 
hardy  but  produce  poor  fruits.  Now  by  grafting  cions  from  the 
trees  bearing  delicious  fruits  on  the  hardy  trees  as  stocks,  one  may 
obtain  individuals  that  are  hardy  and  at  the  same  time  bear 


Fig.  210.  —  Cuttings  of  Sugar  Cane  being  properly  placed  in  the  trenches, 
after  which  they  are  covered  by  dragging  dirt  into  the  trenches.  After 
N.  A.  Cobb. 

delicious  fruits.  Second,  it  enables  one  to  preserve  hud  sports, 
which  are  individual  branches  that  show  qualities  strikingly  dif- 
ferent from  other  branches  of  the  same  plant.  Since  bud  sports 
rarely  take  root  from  cuttings  or  come  true  from  seed,  grafting 
is  usually  the  only  way  of  preserving  them;  and  so  important 
are  bud  sports  that  most  of  the  best  varieties  of  such  fruits  as 
Apples,  Pears,  and  Oranges  have  originated  as  sports,  which,  after 
being  grafted  on  stocks,  became  trees  which  by  further  grafting 
have  been  multiplied. 

Often  minor  influences  of  the  stock  on  the  cion,  such  as  dwarfing, 
hastening  the  fruiting  period,  or  altering  the  time  of  blossoming, 
are  desirable,  and  are  obtained  by  grafting  the  cion  on  suitable 
stocks.  For  example.  Pears  are  dwarfed  and  fruit  at  an  earlier 
age  when  grafted  on  the  Quince.  Apples  are  influenced  in  the 
same  way  when  grafted  on  the  so-called  "  Paradise  "  stock,  a  name 


230      THE   PROPAGATION  OF   PLANTS   BY   MEANS  OF   STEMS 


given  to  certain  surface-rooting  dwarf  varieties  of  Apples.  By 
grafting  Pears  on  Pear  stocks  raised  from  seed  or  by  grafting 
Apples  on  stocks  raised  from  the  seed  of  the  Crab  Apple,  larger 
and  longer-lived  trees,  which  do  not  fruit  so  soon,  are  secured.     It 

is  claimed  that  in  some  cases 
the  quality  of  the  fruit  is 
changed,  having  more  sugar 
or  more  acid  according  to  the 
nature  of  the  stock.  One  of 
the  most  interesting  and  for  a 
long  time  a  very  puzzling  re- 
sult of  grafting  is  the  chimera, 
which  arises  when  a  bud  de- 
velops from  the  wound  callus 
in  such  a  way  that  the  tissues 
of  both  cion  and  stock  grow 
out  together  to  form  the 
branch.  The  tissues  of  the 
members  may  grow  out  side 
by  side,  in  which  case  each 
member  forms  a  side  of  the 
branch,  or  the  tissues  of  the 
members  may  be  so  related 
to  each  other  that  one  mem- 
ber forms  the  core  and  the 
other  the  covering  of  the 
branch.  In  either  case  both  members  may  be  represented  in  the 
leaves,  flowers,  and  fruit  of  the  branch  and  be  the  cause  of  very 
peculiar  combinations  of  characters.  For  example,  in  Apples  one 
side  of  the  fruit  may  be  of  one  variety  and  the  other  side  of  an- 
other variety.  In  grafting  together  Tomatoes  and  the  Black 
Nightshade,  the  latter  of  which  has  small  ])lack  fruits,  chimeras 
in  which  one  member  formed  the  core  and  the  other  the  covering 
of  the  branch  have  been  obtained.  As  a  result  very  queer  fruits 
have  been  produced.  Some  resembled  tomatoes  in  size  but  had 
the  black  skin  of  the  Nightshade  berry,  while  others  were  similar 
in  size  to  the  small  berry  of  the  Nightshade  but  had  the  yellow 
or  red  skin  of  the  Tomato.  Also  in  the  character  of  the  leaves 
and  flowers,  these  chimeras  presented  queer  combinations.  By 
a  study  of  chimeras  produced  experimentally,  as  those  of  the 


Fig.  211.  —  Hardwood  cuttings. 
a,  simple  cutting;  h,  heal  cutting; 
c,  mallet  cutting;  d,  single-eye  cutting. 
After  L.  C.  Corbett. 


BUDDING 


231 


Tomato  and  Nightshade  just  described,  an  explanation  has  been 
obtained  for  some  so-called  graft-hybrids,  one  of  note  being  the 
Cytisus  Adami  which  was  produced  many  years  ago  by  grafting  to- 
gether two  shrubs,  one  having  purple  and  the  other  yellow  flowers. 


Vv>Vivi|-;,;-,-_;;.^ 


Fig.  212.  —  Layering  of  the  grape  vine.     The  vine  has  been  bent  to  the 
ground  and  covered,  and  from  it  roots  and  shoots  are  developing. 

As  a  result  of  this  graft  and  further  grafting,  shrubs  having  some 
branches  bearing  purple  flowers  and  others  bearing  yellow  flowers 
were  obtained.  Even  a  flower  might  be  part  purple  and  part  ye\- 
low.  For  a  long  time  some  thought  these  strange  plants  were  true 
hybrids,  but  now  we  are  quite  sure  that  they  are  only  chimeras. 
Budding.  —  Budding  is  similar  to  grafting,  the  principal  differ- 
ence being  in  the  character  of  the  cion.  In  budding,  instead  of 
twigs  or  branches,  only  a  small  strip  of  bark  bearing  a  bud  is  used. 
Tliis  strip  of  bark,  which  is  cut  so  that  it  has  cambium  on  its 
inner  face,  is  inserted  into  the  young  bark  of  the  stock  in  such 
a  way  that  the  cambiums  can  unite.  A  study  of  Figure  214  will 
show  how  the  bud  is  inserted.  After  a  T-shaped  cut  is  made  in 
the  young  bark  of  the  stock,  the  bark  on  the  edges  of  the  cut  is 
lifted  and  the  cion  is  slipped  in,  the  lifted  bark  on  each  side 
holding  it  in  place.  After  the  cion  is  in  place,  it  is  fastened  more 
firmly  by  wrapping  strings  around  the  stem  just  above  and  below 
the  inserted  bud.  Peaches  arc  quite  commonly  propagated  by 
budding  and  sometimes  Apples,  Pears,  and  other  fruit  trees  are 
propagated  in  this  way. 


232      THE   PROPAGATION   OF  PLANTS  BY  MEANS  OF  STEMS 


lA 


41 


Fig.  213.  —  Cleft  Grafting.     A,  cion;    B,  cions  inserted  in  cleft  of  stock; 
C,  the  wound  covered  with  wax.     After  G.  C.  Brackett. 


Fig.  214.  —  Budding,  a,  opening  of  bark  for  insertion  of  bud;  6,  removing 
the  bud;  c,  inserting  the  bud;  d,  bud  inserted;  e,  bud  properly  wrapped. 
After  G.  C.  Brackett. 


CHAPTER  XI 

LEAVES 

Characteristic  Feature  of  Leaves 

Ordinary  green  leaves,  known  as  foliage  leaves,  may  be  defined 
as  the  food-making  organs  of  the  plant.  The  green  cortex  of 
stems  makes  some  food,  but  usually  the  greater  part  of  it  is  made 
in  the  leaves.  Leaves  are  constructed  especially  for  utilizing  the 
carbon  dioxide  of  the  air  and  the  water  brought  up  from  the  roots 
in  the  manufacture  of  sugar.  Although  starch  may  sometimes 
be  the  first  food  product  formed  by  the  leaves  from  carbon  dioxide 
and  water,  the  evidence  indicates  that  ordinarily  the  first  product 
is  sugar  from  which  starch  is  formed  later.  Sugar  then  may  be 
regarded  as  the  fundamental  plant  food,  since  from  it  or  from  the 
proteins  of  which  it  is  the  chief  element,  plants  build  by  chemical 
transformations  all  of  their  structures  and  organic  materials  of 
whatever  kind.  Leaves  transform  sugar,  when  it  is  abundant, 
into  starch  which  serves  as  a  storage  form  of  sugar.  Although 
proteins  can  be  made  in  any  living  part  of  the  plant,  it  has  been 
demonstrated  that  leaves  are  very  active  in  the  formation  of  this 
food.  Although  proteins  may  be  regarded  as  a  secondary  food 
since  they  depend  upon  sugar  as  their  chief  foundational  ele- 
ment, they  are  exceedingly  important  because  from  them  the  pro- 
toplasm, the  living  substance  of  the  cell,  is  formed.  Proteins, 
although  of  various  kinds,  are  formed  by  combining  chemically 
the  mineral  elements  of  the  soil,  such  as  nitrogen,  sulphur,  and 
phosphorus,  with  the  elements  of  sugar.  Since  leaves  manu- 
facture sugar  and  are  well  supplied  with  the  mineral  elements, 
they  are  well  equipped  for  the  manufacture  of  proteins. 

The  efficiency  of  green  tissue  in  making  sugar  depends  upon 
exposure  to  light  and  air,  and  the  foliage  leaf  may  be  considered 
a  device  for  securing  good  exposure  of  green  tissue.  The  elevat- 
ing of  leaves  into  light  and  air  by  the  stem,  and  their  arrangement, 
position,  form,  and  structure  are  related  to  the  prol:)leni  of 
securing  suitable  exposure,  and  thus  to  food  manufacture. 

233 


234 


LEAVES 


The  variations  in  form  and  structure  of  leaves  is  so  great  that 
they  are  often  used  in  classifying  plants,  and  for  this  purpose 
many  technical  terms  have  been  devised  to  describe  these  varia- 
tions. Since  most  of  these  variations  concern  only  those  who  are 
interested  especially  in  the  classification  of  plants,  only  the  most 
common  ones  will  be  considered  in  this  presentation. 

Primary  and  Secondary  Leaves.  —  Leaves  may  be  divided  into 
primary  and  secondanj.  The  cotyledons  are  examples  of  primary 
leaves.  The  cotyledons  are  parts  of  the 
embryo  and  hence  precede  the  stem  in 
development,  while  the  leaves  developing 
later  and  called  secondary  leaves  arise 
from  the  stem.  The  secondary  leaves  are 
usually  numerous,  while  the  primary  leaves 
are  few  in  number.  Primary  leaves  are 
usually  short  lived  and  often  fall  away  as 
soon  as  their  stored  food  is  exhausted.  Gen- 
erally they  disappear  while  the  plant  is  still 
quite  small.  Consequently  the  leaves  of 
plants  that  attract  attention  are  the  second- 
ary ones,  and  when  the  term  leaves  is  used, 
secondary  leaves  are  usually  meant. 

Development.  —  Leaves  develop  upon  the 

sides   of   the   growing  points   of  stems  and 

15.  —  Leaf  of  first  appear  as  mere  swellings,  the  smallest 

b,  blade;  swellings  being  near  the   apex.     It  follows 


Fig 

the   Apple 


P,  petiole;  s,  stipules;  ^^gn  that  the  oldest  leaves  are  at  the  base 
r,  leaf  base.  ^^  ^^^  ^^^^^  ^^  ^^^.^^     r^^^^^  .^^  ^  ^^^^  ^^^jj,^ 

for  example,  the  leaves  decrease  in  age  from  the  lowest  leaf  on 
the  stalk  to  the  highest.  Swellings  similar  to  those  that  become 
leaves  appear  later  just  above  the  leaf  swellings,  and  these 
become  the  buds  which  appear  in  the  axils  of  the  developed 
leaves.  In  woody  plants  which  prepare  for  a  rest  period,  the 
leaves  are  partly  developed  during  the  previous  season,  and  rest 
in  the  bud  in  a  miniature  form  until  the  following  spring  when 
they  burst  from  the  bud  scales  and  in  a  few  days  complete  their 
development. 

Parts  of  a  Leaf.  — ^  In  a  typical  foliage  leaf,  such  as  that  of  the 
Apple  shown  in  Figure  215,  there  are  three  parts:  the  expanded 
portion  or  blade;  the  leaf  stalk,  called  petiole,  which  supports  the 


LEAF   BLADE 


235 


Fig.  216. 


blade  and  makes  connection  with  the  twig;  and  a  pair  of  small 
leaf-like  appendages  at  the  base  of  the  petiole,  known  as  stipules. 
The  portion  of  the  leaf  at  the  point  of  contact  with  the  twig  or 
stem  is  called  the  leaf  base.  The  leaf  base  is  generally  enlarged 
so  as  to  form  a  sort  of  cushion  by  which  the  leaf  is  attached  to 
the  stem. 

The  leaves  of  most  plants  are  not  typical,  but  have  one  or 
more  parts  lacking.  The  stipules  are  very  frequently  absent. 
The  leaves  of  the  Thistle,  Wild  Let- 
tuce, Mullein,  and  many  other  plants 
have  no  petioles,  the  blade  being 
directly  attached  to  the  stem.  Such 
leaves  are  said  to  be  sessile  (mean- 
ing sitting).  (Fig.  216.)  In  Corn, 
Wheat,  Oats,  and  Grasses  in  general 
the  leaves  have  no  petioles  and  the 
leaf  base  is  much  expanded  and 
enwraps  the  stalk  completely  for  a 
considerable  distance  above  the  node. 
A  leaf  base  enwrapping  or  sheathing 
the  stem  as  just  described  for  the 
Grass  type  of  leaf  is  called  a  leaf  sheath.  At  the  juncture  of 
the  blade  with  the  sheath  in  the  Grass  type  of  leaf  occurs  an 
outgrowth  which  fits  closely  to  the  stem  and  is  known  as  the 
ligule  or  rain  guard.  In  the  Corn  and  some  other  plants  of 
the  Grass  type  small  projections,  known  as  auricles,  occur  at 
the  base  of  the  blade.  {Fig.  217.)  Leaves  designated  as 
'perfoliate  have  their  blades  so  joined  around  the  stem  that 
the  stem  appears  to  pass  through  the  leaf  as  shown  in 
Figure  218. 

Leaf  Blade.  —  In  general,  the  leaf  blade  is  expanded  into  a 
broad  thin  structure;  but  all  gradations  exist  between  such  forms 
and  those  that  are  thick  and  fleshy  or  even  cylindrical. 

The  border  of  the  blade,  called  margin,  may  be  smooth  or 
quite  irregular,  and  the  character  of  the  leaf  margin  is  one  of  the 
features  used  in  classifying  plants.  When  the  margin  is  smooth, 
as  that  of  the  Corn  leaf,  it  is  said  to  be  entire.  Irregular  margins 
differ  much  in  the  form  and  depth  of  the  indentations,  as  illus- 
trated in  Figure  219.  The  margin  may  be  cut  up  by  many  small 
notches,  as  the  margin  of  the  Apple  leaf  shown  in  Figure  215,  or 


—  Sessile  leaf  of  a 
Thistle. 


236 


LEAVES 


Fig.  217.  —  A  portion  of  a  Corn  plant  showing  two  leaves,     a,  loaf  blade; 
s,  leaf  base  called  leaf  sheath;   w,  auricles;  /,  ligule  or  rain  guard. 


Fig.  218.  —  Cup  Plant  {Silphium  perfoliatum),  a  plant  with  perfoUate  leaves. 


EXPOSURE  TO   LIGHT 


237 


the  notches  may  be  very  deep  and  divide  the  ])lade  into  lobes,  as 
the  leaves  of  the  Gooseberry,  Cotton,  Dandelion,  some  Oaks, 
Maples,  and  many  other  plants  illustrate.  In  some  cases  the 
blade  is  so  divided  that  it  is  made  up  of  independent  portions 
united  to  a  common  stalk,  each  independent  portion  being  called 
a  leaflet.     Many  familiar  plants,  such  as  Clover,  Alfalfa,  Vetches, 


Fig.  219.  —  A,  Margins  of  leaves,  a,  serrate; 
d,  undulate;  e,  sinuate.  B,  lobed  leaf  of  Grape, 
leaf  of  Black  Locust.     A,  after  Gray. 


b,   dentate;    c,   crenate; 
C,  pinnately  compound 


the  Walnut,  Ash,  Locust,  and  Sumach,  have  leaves  divided  into 
leaflets.  The  number  of  leaflets  into  which  the  leaves  of  different 
plants  are  divided  varies  widely.  In  the  leaves  of  Clover  and 
Alfalfa  three  leaflets  are  common,  while  leaves  of  the  Black 
Walnut  often  have  twenty  or  more  leaflets.  (Fig.  220.)  Leaves 
divided  into  leaflets  are  said  to  be  corn-pound,  while  those  less 
divided  are  called  simple. 

Leaflets  resemble  simple  leaves  and  in  case  of  some  compound 
leaves  it  is  possible  for  one  to  mistake  the  axis  to  which  the  leaf- 
lets are  attached  for  a  branch  of  the  stem  and  the  leaflets  for 
leaves.  However,  since  buds  occur  only  in  the  axils  of  leaves, 
one  can  tell  whether  the  leaf-like  structure  is  a  leaf  or  a  leaflet 
by  the  presence  or  absence  of  a  bud  in  its  axil. 

Exposure  to  Light.  —  Unless  the  leaf  is  properly  exposed  to 
light,  it  can  not  be  an  efficient  food-maker.  It  is  not  always  a 
problem  of  securing  enough  light,  but  often  one  of  escaping  light 
that  is  too  intense;  for  too  intense  light  often  injures  leaves  and 
consequently  checks  them  in  their  work.     The  adjustment  to 


238 


LEAVES 


light,  therefore,  is  a  delicate  one,  and  many  leaves  do  not  have 
the  proper  amount  of  light. 

The  more  or  less  horizontal  position  which  the  leaves  of  many- 
plants  assume  enables  them  to  receive  the  direct  and  most  in- 
tense rays  on  their  upper  surface.  Leaves  in  this  position  receive 
more  light  rays  than  those  having  the  oblique  or  vertical  position. 


Fig.  220.  —  Leaves  divided  into  leaflets.  A, 
leaflets.  B,  Walnut  leaf  having  many  leaflets, 
stipules;  b,  bud. 


leaf  of  Alfalfa  with  three 
I,  leaflets;    p,  petiole;    s, 


The  separation  of  leaves  through  the  elongation  of  the  internodes 
is  another  means  of  securing  better  exposure.  For  example,  dur- 
ing the  early  growth  of  the  Corn  plant,  the  leaves  are  closely 
packed  around  the  growing  point  of  the  stem  and  only  the  outer 
ends  of  the  blades  are  well  exposed.  But  through  the  elongation 
of  the  internodes,  all  of  the  leaves  are  finally  separated,  so  that  at 
the  time  the  tassel  and  ears  appear  all  portions  of  the  leaves 
receive  light. 

The  way  leaves  are  arranged  on  the  stem  is  a' so  an  important 
feature  in  securing  proper  exposure.  There  are  three  coimnon 
arrangements,  alternate,  opposite,  and  whorled.  In  the  alternate 
arrangement,  there  is  but  one  leaf  at  a  node  and  they  appear  to 
alternate,  first  on  one  side  of  the  stem,  then  on  a  different  side. 


EXPOSURE  TO   LIGHT 


239 


The  leaves  of  Corn  and  other  Grasses  are  good  examples  of  the 
alternate  arrangement.  In  Corn,  for  example,  the  second  leaf 
appears  at  the  next  node  above  and  on  the  opposite  side  of  the 
stem  from  the  first  leaf,  and  the  third  leaf  appears  at  the  third 
node  and  almost  directly  over  the  first  leaf.  Usually  on  account 
of  a  slight  twisting  of  the  stem,  the  leaf  blades  do  not  occur 


Fig.  221.  — Tobacco,  a  plant  with  the  alternate  arrangement  of  leaves. 
After  Hayes. 

directly  over  each  other,  but  extend  in  slightly  different  direc- 
tions, so  that  the  lower  leaves  are  not  directly  in  the  shade  of  the 
upper  ones.  In  fruit  trees  and  many  other  plants  having  the  alter- 
nate arrangement,  the  second  leaf  is  not  quite  on  the  opposite 
side  from  the  first  and  neither  is  the  third  leaf  usually  over  the 
first.  {Fig.  221.)  The  leaves  are  so  arranged  that  no  large  open 
spaces  appear  in  looking  in  from  the  end  of  the  twig  as  shown  in 


240 


LEAVES 


Fig.  222.  —  End  view  of  an  Apple  twig,  showing  the  leaves  alternately 
arranged  and  so  located  in  reference  to  each  other  that  all  receive  light. 


Fig.  223.  —  A  Wild  Sunflower 
with  opposite  arrangement  of 
leaves.     After  Bailey. 


Fig.  224.— Sweethearts  {Galium 
Aparine),  one  of  the  weeds  having 
leaves  in  whorls.  After  Beal. 
Mich.  .Agr.  Exp.  Sta. 


EXPOSURE  TO  LIGHT 


241 


Figure  222.     Many  trees  as  well   as  many   herbaceous  i)lants, 

such  as  Cotton,  Clover,  Alfalfa,  Tomatoes,  Potatoes,  Buckwheat, 

and   Flax,   have  the  alternate   arrangement  of  leaves.     In  the 

opposite  arrangement  two  leaves  appear  at  each  node  on  opposite 

sides  of  the  stem,  and  neighboring 

pairs  are  set  more  or  less  at  right 

angles  to  each  other,  so  that  as  one 

looks  down  from  above  each  pair 

of  leaves  alternates  in  position  with 

the  pair  above  and  with  the  pair 

below  it  as  shown  in  Figure  223. 

The  opposite  arrangement  is  also 

common   among   both  woody  and 

herbaceous  plants.     In  the  whorled 

arrangement  more  than  two  leaves       t-      r>or        t-w     j  i-         •       i 
°  I'iG.  225.  —  Dandelion  viewed 

occur  at  a  node,  as  illustrated  in  from  above.  The  leaves  form  a 
Figure  22Ji..  In  this  arrangement  rosette  and  the  lower  leaves  are 
the  leaves  are  also  so  placed  as  to  ^^^^  logger  than  the  upper  ones. 
shade  each  other  as  little  as  possible. 

In  plants,  like  the  Dandelion  and  Plantain,  which  have  very  short 
stems  bearing  many  leaves,  the  leaves  form  a  mat,  called  a  rosette, 

on  the  surface  of  the 
ground.  It  is  readily  seen 
that  leaves  so  closely 
crowded  as  they  are  in  the 
rosette  must  shade  each 
other  considerably,  but 
they  have  the  advantage 
of  being  exposed  less  than 
those  on  elongated  stems 
to  the  loss  of  water  by 
transpiration.  In  the 
rosette  much  shading  is 
eliminated  b}^  a  difference 
in  length  of  petioles,  for  the  outer  and  under  leaves  of  the  rosette 
have  longer  petioles  which  push  their  blades  beyond  those  of  the 
upper  leaves,  and  in  this  way  they  escape  the  shade  of  the  leaves 
above.  This  feature  is  noticeable  in  the  rosette  of  the  Dandelion 
shown  in  Figure  225.  Another  arrangement  of  leaves  which  is 
favorable  to  light  exposure  is  called  a  leaf  mosaic,  being  so  named 


Fig.  226.  —  Nasturtiums  showing  mosaic 
arrangement  of  leaves. 


242  LEAVES 

from  the  fact  that  the  edges  of  the  leaves,  as  viewed  from  above, 
fit  together  Yike  the  httle  tiles  of  a  real  mosaic.  The  fitting 
together  in  this  way  is  the  best  arrangement  for  the  individual 
leaves  in  a  large  mass  to  receive  light.  (Fig.  226.)  A  general 
mosaic  arrangement  of  leaves  may  be  observed  in  connection  with 
almost  every  broad  leaved  plant,  but  is  most  noticeable  in  the 
Ivies  where  their  mosaic  of  leaves  often  completely  cover  the 
surface  of  a  wall.  In  case  of  stems  exposed  to  direct  light  on 
only  one  side,  as  the  horizontal  branches  of  trees,  and  stems 
prostrate  on  the  ground  or  in  contact  with  a  support,  such  as 
Cucumbers,  Melons,  and  climbing  vines,  the  petioles  of  those 


Fig.  227.  —  Maple  twig,  showing  mosaic  arrangement  of  leaves. 

leaves  on  the  under  side  of  the  stem  usually  curve  so  as  to  bring 
the  blades  to  the  light.  For  example,  in  looking  up  into  a  tree 
in  full  foliage,  one  will  notice  that  the  horizontal  branches  are 
comparatively  bare  underneath,  the  leaf  blades  being  displayed 
on  the  upper  side  as  a  mosaic.     {Fig-  227.) 

When  plants  receive  light  from  only  one  side,  as  plants  grown 
in  a  room  near  a  window,  the  entire  plant  usually  bends  toward 
the  light,  thus  bringing  the  leaf  blades  into  a  better  position  for 
exposure.     {Fig.  228.) 

General  Structure  of  Leaves 

Although  diverse  in  form  and  arrangement,  foliage  leaves 
show  much  uniformity  in  structure,  being  so  constructed  as  to  be 
adapted  to  the  function  of  food-making.     In  general,  they  have 


EXPOSURE  TO  LIGHT 


243 


Fig.  228.  —  Geranium  growing  near  a  window,  toward  wliich  it  is  bending 
and  thereby  bringing  the  leaves  in  a  better  position  in  reference  to  light. 


three  kinds  of  tissues.  First,  there  are  the  conductive  tissues 
which  bring  the  water  and  mineral  salts  to  the  leaf  and  carry- 
away  the  manufactured  foods.  Second,  there  is  the  protective 
tissue  consisting  of  epidermis  which  protects  the  delicate  tissues 
within  the  leaf  against  drying,  intense  light,  the  entrance  of 
destructive  organisms,  and  to  some  extent  gives  rigidity  to  the 


244 


LEAVES 


leaf.  In  some  cases  there  are  special  strengthening  tissues ' 
developed  within  the  leaf,  either  in  connection  with  the  conduc- 
tive tissues  or  separately.  Third,  most  important  of  all  is  the 
food-making  tissue,  known  as  the  mesophyU,  because  it  fills  the 
interior  of  the  leaf.  The  green  mesophyll  is  usually  called  chloren- 
cJujma  because  of  its  green  color. 

The  Conductive  Tissues.  —  The  conductive  tissues  of  leaves 
consist  of  vascular  tissues  similar  to  those  of  the  vascular  bundles 


Fig.  229.  — A,  leaf  of  Solomon's  Seal,  showing  parallel  veining;   B,  leaf  of 
Willow,  showing  net  veining.     After  Ettinghausen. 

of  stems  and  roots.  They  constitute  the  veins.  The  veins  are 
simply  branches  of  the  vascular  bundles  of  the  leaf  trace,  and 
the  leaf  trace  is  a  branch  of  the  vascular  cylinder  of  the  stem. 
Thus  through  the  direct  connection  of  the  vascular  tissues  of  the 
leaves  with  those  of  the  stem,  which  in  turn  are  in  direct  connection 
with  the  vascular  tissues  of  the  roots,  all  parts  of  the  plant  are 
brought  into  close  communication  for  the  exchange  of  materials. 


EPIDERMIS  245 

The  veins  run  through  the  mesophyll  of  the  leaf  and  form  a 
frame-work,  which  with  its  numerous  fine  branches,  known  as 
vemlets,  resembles  a  fine-meshed  net  when  a  leaf  is  held  up  to  the 
light.  The  finest  veinlets  can  be  seen  only  with  the  aid  of  the 
microscope.  It  is  by  this  profuse  branching  of  the  veins  and 
veinlets  that  all  parts  of  the  mesophyll  are  brought  into  direct 
contact  or  close  relation  with  the  conductive  tissues.  Although 
the  larger  veins  are  often  thicker  than  the  leaf  and  form  prominent 
ridges  on  its  under  side,  they  taper  down  to  the  veinlets  which 
are  well  buried  within  the  mesophyll. 

The  charactef  of  the  veining,  known  as  venation,  differs  con- 
siderably in  different  leaves  and  there  are  two  types  of  venation 
of  some  prominence.  {Fig.  229.)  One  is  the  parallel-veined  type, 
in  which  there  are  a  number  of  parallel  principal  veins  with 
obscure  cross  veins.  This  type  is  familiar  in  Corn  leaves  and 
is  characteristic  of  monocotyledonous  plants  in  general.  The 
other  is  the  net-veined  type,  in  which  there  is  one  or  only  a  few 
principal  veins  and  their  branches  so  fork  and  join  each  other 
that  a  quite  noticeable  network  of  veins  and  veinlets  is  formed 
as  Maple  or  Oak  leaves  will  illustrate.  This  type  is  characteristic 
of  Dicotyledons.  Many  leaves  have  one  large  primary  vein 
called  midrib.  Some  leaves  have  a  number  of  primary  veins, 
which  are  then  called  nerves,  and  a  leaf  is  described  as  three- 
nerved,  five-nerved,  or  whatever  the  number  may  be. 

Epidermis.  —  The  epidermis  forms  a  continuous  covering  over 
the  leaf  except  where  it  is  broken  by  the  openings  of  the  stomata. 
The  stomata,  although  microscopical  in  size,  afford  the  openings 
necessary  for  the  exchange  of  gases  between  the  interior  of  the 
leaf  and  the  outside  air.  The  epidermis  is  usually  one  layer  of 
cells  in  thickness,  but  in  some  leaves,  especially  those  of  dry 
1-egions,  it  is  often  thicker.  Except  in  the  cells  of  the  stomata, 
the  epidermis  usually  contains  no  pigments,  although  it  may 
appear  to  have  since  the  green  color  of  the  mesophyll  beneath 
readily  shows  through  it.  Sometimes  the  epidermis  contains  a 
red  pigment,  called  anthocyan,  which  causes  a  part  or  all  of  the 
leaf  to  be  red.  Red  pigment  is  often  noticeable  in  the  leaves  of 
Sorghum  and  is  common  in  some  greenhouse  plants  of  which 
the  Wandering  Jew  is  a  familiar  example.  The  epidermis  when 
smooth  has  the  appearance  of  having  been  greased,  due  to  the 
deposits  of  cutin  in  its  outer  cell  walls.     Cutin  usually  forms  a 


246 


LEAVES 


thin  film  called  cuticle  on  the  outer  surface  of  the  epidermis. 
Being  a  waxy  substance  and  impervious  to  water,  it  makes  the 
epidermis  more  protective  against  the  loss  of  water.  Sometimes, 
as  in  Cabbage,  a  waxy  substance  that  can  be  easily  rubbed  off  is 
deposited  on  the  outside  of  the  epidermis.  Frequently,  as  the 
common  Mullein  and  some  Thistles  illustrate,  the  epidermis 
develops  hairs,  which  are  sometimes  so  long  and  dense  as  to  give 
the  leaf  a  white  woolly  appearance.  Some  leaves,  as  those  of  the 
Mints  illustrate,  have  glands  that  secrete  fluids  to  which  the  odor 
of  the  plant  is  due.  Some  plants  are  cultivated  on  account  of 
the  commercial  value  of  their  glandular  secretions.  In  many 
cases  the  epidermal  secretions  of  leaves,  if  not  unpleasant  to 
the  sense  of  smell,  are  to  the  taste,  and  therefore  may  protect 
plants  against  being  eaten  by  stock.     In  fact  all  of  the  epidermal 

modifications  are  sup- 
posed to  be  related  to  the 
protection  of  the  plant 
in  one  way  or  another. 

Mesophyll.  — The 
mesophyll,  as  the  term 
suggests,  occupies  the 
middle  region  of  the  leaf 
and  its  distinctive  fea- 
ture is  its  green  color 
upon  which  the  power 
to  manufacture  food  de- 
pends. It  is  soft  spongy 
Fig.  230. -a  much  enlarged  surface  view  tissue  and  is  composed 
of  the  lower  epidermis  of  a  Bean  leaf,  a,  of  a  number  of  layers  of 
epidermal  cells;  s,  stomata;  g,  guard  cells;  cells  which  surround  the 
t,  slit-like  opening  between  the  guard  cells  smaller  conductive 
through  which  gases  pass.  ^^.^^^g  ^^^  g^  ^^^  gp^^^g 

between.  It  is  so  delicate  in  structure  and  so  closely  joined 
to  the  epidermis  that  in  most  leaves  it  is  difficult  to  remove  the 
epidermis  without  tearing  away  some  of  the  mesophyll. 


Cellular  Structure  of  Leaves 

To  learn  the  finer  structural  features  of  leaves,  a  microscope 
must  be  employed,  so  that  the  cells  of  the  different  leaf  tissues 
may  be  studied. 


CELLULAR  STRUCTURE  OF   LEAVES 


247 


A  surface  view  of  a  small  portion  of  epidermis  stripped  off  and 
highly  magnified  is  shown  in  Figure  230.  The  epidermal  cells  in 
this  view  are  irregular  in  shape,  but  so  closely  fitted  together 
that  no  openings  occur  except  through  the  stomata.  A  stoma 
(singular  of  stomata)  is  a  definite  structure,  consisting  of  two 
curved  cells,  known  as  guard  cells,  which  are  so  fitted  together 
as  to  enclose  a  slit-like  opening.  The  guard  cells  are  so  named 
because  they  regulate  the  size  of  the  opening.  Some  plants, 
such  as  the  Grasses  of  which  Corn  is 
a  familiar  example,  have  a  peculiar 
type  of  guard  cells  as  Figure  231 
shows.  In  this  case  the  guard  cells 
are  enlarged  at  the  ends,  and  re- 
semble dumbbells  in  shape.  How- 
ever, this  difference  in  shape  seems 
to  have  nothing  to  do  with  their 
behavior,  for  they  open  and  close 
their  slit-like  opening  just  as  the 
ordinary  type  of  guard  cells  is  able 
to  do. 

By  changes  taking  place  within 

the  guard   cells,   the   stomata  are 

opened  and  closed,  but  the  causes 

of  such  changes  are  not  definitely 

known.       The    guard    cells    have 

11  ,      ,  1    ,,  .  -1  Fig.  23L  —  A   much   enlarged 

chloroplasts,  and  there  is  consider-  ^^^^^^  ^^^  ^^  ^^^  ^^.^^^^^^  ^^ 

able  evidence  that  the  chloroplasts  Com,  showing  one  stoma,  g, 
have  something  to  do  with  bringing  guard  cells;  t,  slit-like  opening; 
about  these  changes.  Since  chloro- 
plasts make  sugar  and  have  the 
power  to  transform  sugar  into  starch 
or  starch  into  sugar,  it  is  evident  that  they  can  alter  the  concentra- 
tion of  the  sugar  in  the  cell  sap  and  in  this  way  alter  the  turgor  pres- 
sure of  the  guard  cells.  For  example,  if  the  chloroplasts  of  the 
guard  cells  manufacture  much  sugar  which  is  allowed  to  concen- 
trate in  the  cell  sap,  then  by  the  principle  of  osmosis  the  guard 
cells  draw  in  water  forcibly  and  develop  a  high  internal  pressure 
which  tends  to  expand  them  and  alter  their  shape.  On  the  other 
hand,  if  the  chloroplasts  remove  the  sugar  from  the  cell  sap  by 
changing  it  into  starch,  which  is  insoluble,  the  result  may  be 


e,  epidermal  cells.  The  chloro- 
plasts are  in  the  ends  of  the  guard 
cells. 


248  LEAVES 

that  the  guard  cells  then  tend  to  shrink  through  lack  or  loss  of 
water,  since  their  power  to  draw  in  and  retain  water  decreases 
with  the  loss  of  dissolved  substances  from  their  cell  sap.  Re- 
gardless of  what  the  chloroplasts  have  to  do  with  it,  it  is  obvious 
that  when  the  guard  cells  are  swollen  with  water  they  bow  out, 
that  is,  curve  away  from  each  other  and  make  the  slit  larger. 
On  the  other  hand,  when  the  guard  cells  are  shrunken  through 
the  loss  of  water,  they  straighten  and  make  the  slit  smaller. 
Hence  the  stomata  tend  to  open  when  the  water  supply  is  abun- 
dant and  close  when  water  is  scarce. 

The  importance  to  the  plant  of  closing  the  stomata  when  water 
is  scarce  is  apparent,  for  much  water  can  be  lost  through  open 
stomata.  It  would  seem,  therefore,  that  the  guard  cells  regulate 
the  loss  of  water  from  the  plant  and  this  they  do  to  some  extent. 
However,  it  has  been  found  that  stomata  open  in  light  and  close 
in  dark,  and  this  tendency  of  light  to  open,  conflicts  with  the 
tendency  of  water  shortage  to  close  them;  for  it  is  during  bright 
hot  daytime  when  the  light  stimulus  to  open  is  probably  strongest, 
that  there  is  the  greatest  shortage  of  water.  That  the  guard 
cells  open  and  close  just  when  they  should  in  order  to  control 
water  loss  is  much  doubted.  The  most  important  feature  of 
stomata  is  that  they  permit  exchange  of  gases. 

Leaves  having  the  horizontal  position  have  their  stomata  much 
more  abundant  on  the  under  surface;  often  they  are  not  found  at 
all  on  the  upper  surface.  On  leaves  that  stand  more  or  less  erect, 
as  those  of  the  Grass  family  and  Carnations,  the  stomata  are 
about  equally  distributed  on  both  sides,  and  on  leaves  which  lie 
on  the  surface  of  the  water,  like  those  of  the  Water  Lily,  they 
occur  only  on  the  upper  side.  The  location  of  the  stomata  on 
the  under  surface  of  horizontal  leaves  is  an  advantage  to  the 
plant,  since  here  the  stomata  are  less  likely  to  become  choked 
with  water  during  rains,  and  also  less  water  is  lost  through  them 
by  evaporation. 

The  number  of  stomata  varies  much  with  different  plants,  but 
about  sixty  thousand  to  the  square  inch  is  a  fair  average.  On 
the  leaves  of  some  plants  there  may  be  as  many  as  four  hundred 
thousand  to  the  square  inch.  In  the  table  on  the  next  page  are 
given  the  number  of  stomata  found  on  a  square  millimeter  of  leaf 
surface  of  some  common  plants. 


CELLULAR  STRUCTURE   OF  LEANES 


249 


NUMBER  AND   DLSTRIBUTlON  OF  STO^L\TA   PER  SQUARE 
MILLIMETER  OF  LEAF  SURFACE 


Lower  Surfaee 

Uppe 

r  Surface 

330 

0 

160 

160 

281 

40 

130 

12 

160 

0 

269 

28 

1          27 

i 

48 

)          23 

1 

25 

f        158 
t         68 

) 

94 

52 

Lilac  (Syringa  vulgaris) 

Alfalfa  (Medicago  sativa) 

Bean  (Phaseolus  vulgaris) 

Tomato  (Lycopersicum  esculentum) 

Cherry 

Pumpkin  (Cucurbita  pepo) 

Oats  (A vena  sativa) 

Corn  (Zea  Mays) 


Although  stomata  are  most  numerous  on  leaves,  they  occur  in 
Flowering  Plants  wherever  there  is  green  tissue  to  be  supplied 
with  gases.  They  are  common  on  fruits,  green  twigs  of  trees, 
and  are  present  on  nearly  all  parts  of  the  aerial  stems  of  herba- 
ceous plants.  On  the  older  twigs  and  trunks  of  trees,  the  stomata 
are  represented  by  the  lenticels  which  are  the  structures  into 
which  stomata  are  transformed  as  the  stem  becomes  enclosed  in 
bark.  The  stomata  are  distorted  and  transformed  into  lenticels 
partly  by  the  stretching  of  the  bark  and  partly  by  the  tissue 
which  grows  up  from  beneath  and  crowds  into  the  stomatal 
openings. 

In  order  to  get  a  view  of  the  epidermis  in  cross  section  and  to 
study  the  chlorenchyma  and  veins  of  a  leaf,  a  thin  section  must 
be  made  and  highly  magnified  as  shown  in  Figure  232.  In  this 
view  an  ordinary  epidermal  cell  is  rectangular,  has  a  large  central 
cavity  separated  from  the  cell  walls  by  only  a  thin  layer  of  proto- 
plasm, and  has  the  outer  wall  more  thickened  than  those  within. 
The  continuity  of  the  epidermis  is  interrupted  by  the  stomata, 
each  of  which  opens  into  an  air  chamber  in  the  mesophyll  just 
beneath. 

The  chlorenchyma  is  composed  of  thin-walled  cells,  having 
thin  layers  of  protoplasm  in  which  the  characteristic  green  bodies 
(chloroplasts)  are  located.  In  most  horizontal  leaves,  the  cells 
of  the  chlorenchyma  are  differentiated  into  two  distinct  groups, 
the  palisade  and  the  spongij  tissue.  The  palisade  tissue  is  next  to 
the  upper  epidermis  and  consists  of  one  or  more  rows  of  compact 
elongated  cells  in  which  chloroplasts  are  especially  abundant. 


250 


LEAVES 


In  leaves  having  an  oblique  or  vertical  position,  palisade  tissue 
may  be  present  also  on  the  lower  side.  The  spongy  tissue,  having 
fewer  chloroplasts  and  so  characterized  on  account  of  its  loose 
structure,  occupies  the  region  between  the  palisade  tissue  and 
lower  epidermis  or  the  region  between  the  palisade  tissues  when 
there  is  a  lower  palisade  tissue  present.     It  consists  of  cells  irregu- 


W       V 

Fig.  232.  —  Cross  section  of  a  Tomato  leaf,  e,  upper  epidermis;  c,  cuti- 
cle; p,  palisade  cells;  s,  spongy  cells;  d,  lower  epidermis;  st,  stoma;  g,  guard 
cells  of  the  stoma;  h,  stomatal  chamber;  v,  vein;  w,  parenchyma  sheath  of 
the  vein.  The  small  bodies  shown  in  the  pahsade  and  spongy  cells  are  the 
chloroplasts. 

lar  in  shape  and  so  loosely  joined  as  to  provide  a  system  of  air 
spaces  which  extend  in  all  directions  reaching  from  the  stomata 
into  the  palisade  tissues.  In  function,  which  is  the  manufacture 
of  food,  the  palisade  and  spongy  mesophylls  are  identical. 

Structurally  chlorenchyma  cells  are  well  adapted  to  their 
function.  Their  thin  cellulose  walls  permit  water  and  sub- 
stances in  solution  to  pass  in  or  out  readily.  They  have  proto- 
plasm, which,  as  in  all  living  cells,  is  the  substance  endowed  with 
life  and,  therefore,  able  to  regulate  its  activities.  The  cytoplasm 
(the  name  applied  to  all  of  the  protoplasm  except  the  nucleus) 
only  partially  fills  the  cell  cavity,  forming  only  a  peripheral 
layer.  In  this  peripheral  layer  the  nucleus  and  also  the  chloro- 
plasts  are  located.     Such   an  arrangement  of  the   protoplasm 


CELLULAR  STRUCTURE  OF  LE.W^ES 


251 


places  the  chloroplasts  around  the  cell  wall  where  they  are  well 
exposed  to  light,  and  provides  a  large  central  vacuole  which 
accommodates  a  large  quantity  of  cell  sap  consisting  of  water  in 
which  sugar,  carbon  dioxide,  oxygen,  mineral  salts,  and  other 
substances  related  to  the  activities  of  the  cell  are  dissolved. 
{Fig.  233.)  Through  the  layer  of  protoplasm,  the  outer  border 
of  which  behaves  as  an  osmotic  membrane,  the  cell  sap  osmoti- 
cally  pulls  in  water  from  the  veins  or  surrounding  cells,  and  in 
this  way  develops  a  pressure  which  dis- 
tends and  gives  rigidity  to  the  cell.  Its 
cells  being  rigid,  the  leaf  is  rigid  and  ex- 
panded to  the  light.  That  this  pressure 
or  turgor  within  the  cells  gives  rigidity 
to  the  leaf  is  shown  by  the  fact  that  leaves 
wilt  when  water  is  so  scarce  that  the  cells 
can  not  maintain  their  internal  pressure. 

The  chloroplasts,  usually  oval  in  shape 
in  Flowering  Plants,  consist  of  two  sub- 
stances. First,  the  chloroplast  has  a  body 
which  consists  of  cytoplasm  denser  than 
ordinary  cytoplasm  and  known  as  a 
plastid.  Plastids  multiply  by  constrict- 
ing into  two  equal  parts,  and  are  as  color- 
less  as    cytoplasm 

pigments.  Second,  there  is  the  chloro- 
phyll which  is  the  green  pigment  that  protoplasm  (p)  containing 
saturates  the  plastid,  which  is  then  known  the  nucleus  (n)  andchloro 
as  a  hloroplastid  or  by  the  shorter  term  pl^ts^(c, 
chloroplast.  In  the  higher  plants  the 
chlorophyll  is  developed  by  the  plastids  and  does  not  occur 
except  in  connection  with  these  bodies.  Plastids  are  common 
in  all  parts  of  the  plant.  In  regions  where  they  develop  no 
pigments,  the  formation  of  starch  from  the  sugar  present  is  their 
chief  function.  They  are  even  abundant  in  underground  organs, 
such  as  fleshy  stems  and  roots,  which  store  starch. 

The  presence  of  chlorophyll  depends  mainly  upon  exposure 
to  light.  That  chlorophyll  disappears  in  the  absence  of  light  is 
well  demonstrated  by  the  fact  that  leaves  lose  their  green  color 
when  light  is  excluded  for  a  time.  Thus  Grass  under  a  board 
or  covered  w:th  dirt  becomes  yellow.     On  the  other  hand,  when 


T  ,,  ,        1  Fig.     233.  —  Chloren- 

unless    they   develop     ,  n   r    ,    r    u 

^,  ,^       ^    cnyma  cell  of  a  leai,  snow- 

ing wall  (w)  and  layer  of 


is  the  large 
central  vacuole. 


252  LEAVES 

leaves  lacking  chloropliyll,  as  those  of  plants  allowed  to  develop 
in  the  dark,  are  brought  to  the  light,  chlorophyll  develops.  Even 
some  underground  structures,  as  Potato  tubers,  will  develop  chlo- 
rophyll when  exposed  to  the  sun.  Hence  the  development  of 
chlorophyll  as  well  as  its  functioning  depends  upon  the  presence 
of  light.  Although  the  body  of  the  chloroplast  can  make  starch 
regardless  of  the  presence  of  pigment  or  light,  its  power  to  make 
sugar  depends  upon  the  presence  of  chlorophyll  and  light. 

The  veins  in  cross  section  show  as  colorless  often  glistening  areas 
in  the  mesophyll.  In  the  central  region  of  a  vein  are  the  two 
conductive  tissues,  the  xylem  and  phloem.  The  xylem,  consisting 
of  large,  empty,  tube-like  vessels  with  spiral,  annular,  and  other 
kinds  of  thickenings  in  their  walls,  occupies  the  upper  region  of  the 
vein.  The  xylem  carries  the  water  and  mineral  elements  to  the 
leaf  tissues.  In  the  lower  region  of  the  vein  is  the  phloem  made 
up  of  small  thin-walled  cells.  The  phloem  carries  away  the 
proteins  and  some  of  the  sugar  made  by  the  leaves.  The  bundle 
sheath,  consisting  of  a  chain  of  cells  having  large  cavities  and  well 
adapted  to  conduction,  forms  a  sheath-like  covering  around  the 
vein.  Through  the  bundle  sheath  much  of  the  sugar  is  carried 
away  from  the  leaf. 

The  Manufacture  of  Food  by  Leaves 

Sugar,  starch,  and  proteins  are  formed  in  leaves,  but  it  is  the 
manufacture  of  sugar  that  is  the  special  function  of  leaves. 
There  are  various  kinds  of  sugar,  but  there  is  considerable  evidence 
that  grape  sugar,  having  the  formula  C6H12O6,  is  the  chief  one 
formed  in  leaves.  From  this  sugar  as  a  basis  other  kinds  of  sugar, 
of  which  cane  sugar  (C12H22O11)  is  a  common  one,  can  be  formed  by 
minor  chemical  changes.  The  formation  of  grape  sugar  is  a  syn- 
thetic process  and,  since  light  is  necessary,  the  process  is  called 
j)hotosynthesis. 

Of  all  plant  processes,  photosynthesis  is  the  most  important,  for 
upon  sugar  as  an  indispensable  constituent  the  formation  of  other 
kinds  of  food  either  directly  or  indirectly  depends.  Thus  without 
the  formation  of  sugar,  such  foods  as  starch,  fats,  and  proteins 
could  not  be  formed,  and  consequently  neither  plants  nor  animals 
could  exist.  In  considering  photosynthesis  there  are  two  main 
topics:  first,  the  nature  of  the  process  in  reference  to  the  mate- 
rials used,  the  work  of  the  chloroplasts,  and  the  function  of  light; 


THE   MANUFACTURE  OF   FOOD   BY  LEAVES  253 

and  second,  the  various  factors  which  modify  the  rate  of  photo- 
synthesis. 

As  the  student  ah-eady  knows,  carbon  dioxide  and  water 
furnish  the  elements  from  which  sugar  is  synthesized.  Th3 
carbon  dioxide  is  obtained  from  the  air  through  the  stomata, 
while  the  water  is  brought  up  from  the  roots  through  the  vascular 
system,  which  through  its  numerous  fine  divisions  in  the  meso- 
phyll  supplies  either  directly  or  indirectly  all  of  the  chlorenchyma 
cells.  The  carbon  dioxide  is  dissolved  in  the  water  with  which 
it  passes  into  the  cells  and  comes  in  contact  with  the  chloroplasts 
where  the  photosynthetic  process  takes  place.  The  details  of 
the  process  involved  in  forming  sugar  from  carbon  dioxide  and 
water  are  not  well  known;  but,  leaving  out  the  intermediate  steps, 
the  equation  6CO2  +  6H2O  =  CeHjoOe  +  6O2  represents  the 
nature  of  the  process.  From  the  equation  it  is  seen  that  there 
are  as  many  molecules  of  oxygen  liberated  as  molecules  of  carbon 
dioxide  used.  Whether  all  or  only  a  part  of  the  6O2  liberated 
for  each  molecule  of  sugar  formed  comes  from  the  carbon  dioxide 
is  not  known.  It  is  possible  that  only  the  carbon  of  the  carbon 
dioxide  is  used,  in  which  case  all  of  the  oxygen  liberated  comes 
from  the  carbon  dioxide,  or  it  may  be  that  both  water  and  carbon 
dioxide  have  their  constituents  dissociated  and  some  oxygen  from 
each  is  included  in  the  6O2. 

Since  photosynthesis  removes  carbon  dioxide  from  the  air  to 
which  it  returns  an  equal  amount  of  oxygen,  it  is  obvious  that 
it  purifies  the  air  and  makes  it  more  wholesome  for  animal  life; 
for  animals  in  their  respiration  use  oxygen  and  liberate  carbon 
dioxide,  which,  if  allowed  to  accumulate,  becomes  injurious  to 
animals.  Not  only  ordinary  respiration  of  both  plants  and 
animals  but  also  fermentation,  ordinary  combustion,  and  all 
other  processes  which  use  oxygen  and  liberate  carbon  dioxide 
have  their  effects  on  the  air  counteracted  by  photosynthesis.  On 
the  other  hand,  the  oxidation  processes  maintain  the  supply  of 
carbon  dioxide  for  photosynthesis.  Thus  photosynthesis  and 
the  oxidation  processes  tend  to  support  each  other. 

Photosynthesis  takes  place  in  the  chloroplast,  but  the  exact 
function  of  either  the  body  or  the  chlorophyll  of  the  chloroplast 
is  not  known.  It  is  generally  believed  that  the  chief  function  of 
the  chlorophyll  is  to  provide  energy  for  the  process;  and  this  it 
docs  by  transforming  the  sun's  rays  into  available  forms  of 


254  LEAVES 

energy.  The  need  of  energy  for  photosynthesis  is  easy  to  under- 
stand. The  combining  of  the  elements  of  carbon  dioxide  and 
water  into  sugar  is  preceded  by  a  process  of  dissociation  in  which 
carbon  dioxide  and  probably  water  are  in  part  at  least  separated 
into  their  elements.  But  carbon  dioxide  and  water  are  very 
stable  compounds,  and  to  separate  them  into  their  atoms  requires 
much  energy.  To  force  the  atoms  of  CO2  to  separate  requires  an 
energy  expressed  by  a  temperature  of  1300°  C.  It  is  obvious 
that  sunlight  will  not  decompose  carbon  dioxide  and  water;  for, 
if  so,  these  elements  would  be  decomposed  in  the  air.  Therefore, 
the  chlorophyll  must  change  the  sun's  rays  into  a  form  of  energy 
which  is  available  for  bringing  about  these  dissociations.  How- 
ever, this  energy  consumed  in  bringing  about  these  dissocia- 
tions is  not  lost,  but  is  stored  in  the  sugar  as  latent  energy  to  be 
released  when  the  sugar  or  the  compounds  formed  from  sugar 
are  broken  into  simpler  compounds  or  into  carbon  dioxide  and 
water.  Thus  another  relation  of  photosynthesis  to  respiration 
and  other  oxidation  processes  now  appears.  Photosynthesis 
stores  the  sun's  energy  in  chemical  compounds  which,  when 
broken  into  simpler  compounds  by  respiration,  become  a  source 
of  energy  for  all  other  plant  or  animal  activities.  It  is  also  the 
sun's  energy  that  is  released  when  coal,  wood,  oil,  and  other  plant 
or  animal  products  are  burned.  Thus  the  chloroplasts,  enabled 
by  their  chlorophyll  to  utilize  the  sun's  energy,  stand  out  as  the 
plant  structures  upon  which  our  supply  of  both  food  and  energy 
depends. 

The  utilization  of  only  certain  rays  of  the  sun  accounts  for  the 
color  of  leaves.  When  chlorophyll  is  boiled  out  of  leaves  with 
alcohol  and  the  solution  is  viewed  with  a  spectroscope,  it  is  seen 
that  the  red  and  blue  rays  are  absorbed  while  most  of  the  green 
rays  are  allowed  to  pass  through.  This  experiment  demonstrates 
that  chlorophyll  uses  the  red  and  blue  rays  for  energy  and  allows 
the  green  rays  to  escape.  Thus  leaves  are  green  because  from 
them  only  green  rays  come  to  our  eyes. 

By  imagining  a  chloroplast  as  a  factory,  the  process  of 
photosynthesis  may  be  summarized  in  the  following  way:  the 
chlorophyll  is  the  machinery  by  which  sunlight,  the  source  of 
power,  is  applied  to  the  work;  carbon  dioxide  and  water  are  the 
raw  materials;  sugar  is  the  product  synthesized;  and  oxygen  is 
a  by-product.     The  veins  are  the  lines  of  transportation  which 


THE   MANUFACTURE  OF   FOOD   BY  LEAVES  255 

bring  up  the  water  from  the  roots  and  carry  away  the  manufac- 
tured products  to  all  parts  of  the  plant. 

The  formation  of  starch,  although  common  in  leaves,  does  not 
depend  upon  the  presence  of  light  except  in  so  far  as  light  is 
necessary  in  providing  sugar;  for  starch  is  formed  abundantly  in 
many  roots,  tubers,  and  other  structures  where  light  is  excluded. 
Starch,  as  its  formula  (C6Hio05)n  shows,  is  very  similar  to  sugar  of 
which  it  is  considered  a  storage  form.  Consequently  its  abun- 
dance in  leaves  where  sugar  is  being  formed  is  to  be  expected. 
Sugar  is  changed  to  starch  not  only  to  make  room  for  more  sugar, 
but  also  to  prevent  injuries  that  may  result  from  its  accumulation. 
According  to  the  laws  of  osmosis,  as  the  sugar  content  of  the  cell 
sap  of  the  chlorenchyma  cells  increases,  their  internal  pressure 
increases.  Consequently  when  the  chloroplasts  are  very  active, 
the  changing  of  the  sugar  into  starch,  which  is  insoluble  in  the 
cell  sap,  is  necessary  to  prevent  the  internal  pressure  of  the 
chlorenchyma  cells  from  becoming  so  high  that  there  is  danger 
of  bursting. 

The  transformation  of  sugar  into  starch  not  only  prevents  the 
accumulation  of  the  sugar  from  interfering  with  the  process  of 
photosynthesis,  but  also  enables  the  plant  to  have  in  storage 
food  which  can  be  drawn  upon  when  conditions  are  unfavorable 
for  photosynthesis.  Thus  at  night  when  photosynthesis  is  inac- 
tive, the  starch  in  the  leaves  is  changed  to  sugar  and  carried  to  those 
regions  where  it  is  needed  for  growth,  and  in  this  way  the  plant 
is  able  to  maintain  its  growth  at  night  as  well  as  in  the  daytime. 

However,  starch  is  not  stored  in  all  parts  of  the  plant  so 
temporarily  as  in  foliage  leaves.  In  some  organs,  such  as 
seeds,  fleshy  roots,  tubers,  and  stems  of  trees,  starch  is  stored  to 
remain  as  a  food  supply  for  next  season's  growth.  Since  the 
starch  stored  in  all  parts  of  the  plant  is  transformed  sugar  which 
is  made  mostly  in  the  leaves,  the  dependence  of  such  structures 
as  seeds,  roots,  and  tubers  upon  leaves  is  obvious;  for  it  is  only 
as  the  leaves  supply  the  sugar  that  these  storage  structures  can 
form  starch. 

The  amount  of  starch  formed  in  foliage  leaves  is  closely 
related  to  the  rate  of  photosynthesis.  In  general,  the  more 
active  the  process  of  photosynthesis,  the  greater  the  amount  of 
starch  formed.  For  this  reason  the  amount  of  starch  present  in 
leaves  can  be  used  in  determining  the  rate  of  photosynthesis. 


256 


LEAVES 


Starch  occurs  in  the  form  of  starch  grains,  which  are  light  in 
color  and  have  a  characteristic  shape  and  structure  as  shown  in 
Figure  234-  When  starch  grains  are  treated  with  iodine,  they 
turn  dark  blue,  and  this  color  test  can  be  applied  directly  to  the 
leaf  to  indicate  the  amount  of  starch  present  and,  therefore,  the 
rate  of  photosynthesis.  In  applying  the  test,  the  leaf  is  first 
treated  with  hot  alcohol  to  remove  the  chlorophyll.     The  leaf, 


Fig.  234.  —  Starch  grains  from  a  Potato  tuber.  A,  simple  grain;  B, 
half-compound  grain;  C  and  D  compound  grains,  c,  hilum.  Enlarged 
540  times.     After  Strasburger. 


now  almost  white,  is  immersed  in  the  iodine  solution  which  turns 
it  blue,  if  starch  is  present,  with  the  depth  of  blue  roughly  indicat- 
ing the  amount  of  starch  present.  If  no  starch  is  present,  then 
the  leaf  takes  only  the  brownish  color  of  the  iodine  solution. 
This  test  is  of  considerable  service  in  experiments  on  photosyn- 
thesis as  its  application  in  Figure  235  shows. 

Proteins  are  made  in  leaves,  but  in  what  part  of  the  leaf 
they  are  made  is  not  known.  The  main  evidence  that  they 
are  formed  in  leaves  is  that  large  quantities  of  them  are  being 
continuously  carried  away  through  the  veins  to  the  stem.  That 
light  is  essential  in  the  formation  of  proteins  is  doubtful,  for 
there  is  considerable  evidence  that  the  energy  employed  in  their 
synthesis  comes  from  chemical  action  and  not  directly  from  sun- 
light.    Although  proteins  are  of  many  kinds,  all  are  formed  by 


FACTORS   INFLUENCING   PHOTOSYNTHESIS 


257 


combining  the  elements  of  sugar,  which  is  the  foundational  sub- 
stance, with  nitrogen,  sulphur,  and  phosphorus  derived  from  the 
mineral  salts  of  the  soil.  Even  if  the  construction  of  proteins  in 
leaves  does  not  depend  upon  light,  it  is  obvious  that  leaves  are 
well  equipped  for  such 
work,  since  they  manufac- 
ture sugar  and  the  water 
brought  up  from  the  soil 
supplies  them  with  an 
abundance  of  mineral  salts. 

Factors  Influencing 
Photosynthesis.  —  The 
factors  influencing  photo- 
synthesis are  light,  temper- 
a  t  u  r  e,  moisture,  and 
amount  of  chlorophyll. 

That  light  is  absolutely 
essential  for  photosynthesis 
is  easily  demonstrated  by 
applying  the  iodine  tests  to 
two  sets  of  leaves  after  one 
set  has  been  kept  in  the 
dark  and  the  other  in  the 
light  for  a  few  days.  Even 
by  shading  only  a  portion 
of  a  leaf  the  necessity  of 
light  for  photosynthesis  can  ^ig.  235.  —  A  leaf  showing  the  relation 
be  demonstrated  as  shown  f^  Photosyiithesis  to  light,  as  indicated  by 
17         u  amount  of  starch  formed.     After  cover- 

m  Figure  23d.  h  or  photo-  i^g  ^^g  ^rea  represented  by  the  light  band, 
synthesis  sunlight  is  best,  the  leaf  was  left  exposed  to  the  sunlight  for 
although  some  photosyn-  a  few  hours,  then  removed  from  the  plant 
thesis   will    take    place    in  ^"^  *^e  ^^^^^  ^^^^  applied.    The  area  pro- 

,n   ■   1    T   1,     ,1     ,     1  tected  has  no  starch  while  the  areas  exposed 

artmcial  light  that   has   a  .>.    j    i    j     ^   xi,  r        i 

.  .  are  qmte  dark,  due  to  the  presence  oi  much 

suitable  intensity.     It  has  gt^rch.    Adapted  from  Palladin. 
been  demonstrated  in 

greenhouses  that  some  plants,  at  least,  carry  on  photosynthesis  at 
night  if  the  proper  kind  of  electric  light  is  provided.  For  many 
plants  the  direct  rays  of  the  sun  are  too  intense,  in  which  case 
photosynthesis  is  most  active  in  strong  diffuse  light.  It  is 
partly  for  this  reason  that  Pineapples,  Tobacco,  Potatoes,  Cotton, 


258  •      LEAVES 

Lettuce,  and  some  other  plants  grow  better  in  some  localities 
under  the  shade  afforded  by  slats  or  light  cotton  cloth.  In  green- 
houses during  the  summer  months  it  is  usually  necessary  to 
protect  the  plants  against  the  intense  rays  of  the  sun  either  by 
painting  the  glass  or  by  some  other  means.  Of  course  in  shading 
plants  not  only  more  favorable  light  for  photosynthesis  is  often 
provided,  but  the  plants  are  also  benefitted  by  being  protected 
from  intense  heat,  excessive  evaporation,  and  from  hail  and  winds. 
In  many  plants,  as  those  of  the  Grass  family,  which  seem  to  thrive 
well  under  the  direct  rays  of  the  sun,  the  surfaces  of  the  leaves 
slant  so  as  to  shun  the  intensity  of  the  direct  rays. 

On  the  other  hand,  it  is  very  common  for  leaves  to  be  so 
situated  that  they  do  not  receive  enough  light.  This  is  commonly 
true  of  the  lower  leaves  of  the  small  grains,  Clover,  Alfalfa,  and 
other  plants  grown  in  thick  stands.  Often  the  leaves  on  the 
interior  branches  of  trees  do  not  receive  sufficient  light.  It  is 
for  this  reason  that  fruit  trees  with  open  heads  have  better  light 
relations  for  their  interior  branches  than  is  afforded  by  trees  with 
a  compact  head. 

Plants  growing  in  the  house  are  usually  insufficient!}'  lighted, 
especially  if  they  are  not  very  near  a  window.  The  problem  of 
overcoming  so  far  as  possible  the  insufficient  lighting  in  green- 
houses during  the  winter  months  is  of  chief  concern  in  greenhouse 
construction,  determining  largely  the  quality,  thickness,  and 
shape  of  the  glass,  and  the  nature  of  the  frame. 

What  should  be  considered  active  photosynthesis,  as  deter- 
mined by  the  amount  of  starch  produced  per  unit  of  time,  varies 
widely  with  different  plants.  However,  investigations  show  that 
a  number  of  plants  can  produce  1  gram  of  starch  per  square 
meter  of  leaf  surface  per  hour  under  conditions  favorable  to  active 
photosynthesis.  At  this  rate  a  leaf  area  of  a  square  meter  can 
produce  10  grams  of  starch  in  a  day  of  10  hours.  To  do  this,  all 
of  the  carbon  dioxide  would  be  taken  from  250  cubic  meters  of 
air.  Carrying  the  calculation  further  in  regard  to  the  use  of 
carbon  dioxide,  it  has  been  estimated  that  a  yield  of  300  bushels 
of  potatoes  on  an  acre  involves,  including  tops  and  all,  about 
5400  pounds  of  dry  substance,  and  to  form  this,  all  of  the  carbon 
dioxide  over  this  acre  to  a  height  of  1|  miles  would  be  used, 
provided  no  carbon  dioxide  were  added  to  the  air  in  the  mean- 
time.    This  estimate  emphasizes  the  importance  of  respiration, 


FACTORS   INFLUENCING   PHOTOSYNTHESIS  259 

combustion,  and  all  oxidation  processes  in  maintaining  the  supply 
of  carbon  dioxide  for  photosynthesis.  Roughly  estimated,  150 
square  meters  of  leaf  area  will  use  up  in  one  summer  all  of  the 
carbon  dioxide  which  an  average  man  produces  through  respira- 
tion in  one  year. 

When  one  considers  that  the  amount  of  carbon  dioxide  in  the 
air  is  only  about  0.03  per  cent,  that  is,  about 
3  parts  in  10,000  parts  of  air,  it  is  surpris- 
ing that  plants  can  make  sugar  as  rapidly 
as  they  do.  Sometimes,  as  around  cities 
with  many  factories,  the  per  cent  of  carbon 
dioxide  may  be  a  little  higher  but  it  is 
always  exceedingly  low.  Of  course  carbon 
dioxide  is  present  in  solution  in  the  soil 
water;  but  it  is  easily  demonstrated  that 
this  carbon  dioxide  is  of  practically  no  help 
to  plants  in  photosynthesis.  To  compensate 
for  the  limited  amount  of  carbon  dioxide, 
it  is  obvious  that  leaves  need  broad  surfaces 
and  a  thorough  distribution  of  chlorophyll, 
so  that  their  'absorbing  surface  may  be 
large.  However,  with  all  of  these  adjust- 
ments of  the  plant,  it  has  been  demonstrated  -p  ^  ^  2  3  6  —  L  f 
that  the  normal  supply  of  carbon  dioxide  showing  the  effect  on 
is  often  insufficient  for  the  maximum  photosynthesis  of  clos- 
amount  of  photosynthesis;  for  some  plants,  ing  the  stomata.  The 
when  surrounded  by  air  in  which  the  stomata  on  the  under 
,      e         1  1  •      •  1      •      •  1  surface  of  the  white  area 

amount  oi  carbon  dioxide  is  increased  up  i      .  u 

^  were  closed  by  covering 
to  1  per  cent,  show  a  corresponding  rise  in  the  epidermis  with  vase- 
photosynthetic  activity.  hue,    thus    fiUing    the 

Since  stomata  are  the  openings  through  stomata  and  excluding 
which  carbon  dioxide  enters  the  leaf,  their  ^^^^o"  dioxide.  After 
number  per  area  of  leaf  surface  and  the 
extent  to  which  they  are  open  affect  the  amount  of  this  gas  that 
reaches  the  mesophyll.  That  photosynthesis  is  inhibited  when 
stomata  are  closed  is  demonstrated  by  the  experiment  shown  in 
Figure  236.  The  experiment  shows  the  necessity  of  keeping  the 
stomata  free  from  dust  and  other  bodies,  such  as  spores  of  plants 
and  secretions  of  insects,  that  clog  the  stomatal  openings.  It  is 
for  this  reason  that  we  are  advised  to  cover  house  plants  with  a 


260  LEAVES 

thin  cloth  while  sweeping.  Also  for  this  reason  it  is  well  to  spray 
with  clean  water  or  even  wash  the  leaves  of  plants  with  clean 
rags,  so  as  to  open  any  stomata  that  may  be  clogged.  Plants 
are  often  much  injured  by  the  clogging  of  their  stomata,  as  in 
case  of  hedges  along  roadsides  or  plants  around  cement  factories. 

As  for  other  plant  processes,  there  is  an  optimum  temperature 
at  which  photosynthesis  is  most  active,  and  above  or  below  this 
temperature  photosynthesis  diminishes.  The  optimum  tempera- 
ture, although  varying  considerably  for  different  plants,  is  not 
far  from  80°  (Fahrenheit)  for  most  plants  in  our  region.  Tem- 
peratures unfavorable  for  photosynthesis  not  only  affect  the 
yield  of  crops  but  also  may  lengthen  the  time  required  for 
maturity,  as  in  case  of  Corn  when  the  summer  is  cool. 

Since  water  is  one  of  the  materials  for  making  sugar,  it  must 
be  present  in  sufficient  quantities  to  supply  this  demand.  Fur- 
thermore, the  lack  of  water  tends  to  cause  the  stomata  to  close 
and  may  thereby  diminish  the  amount  of  carbon  dioxide  entering 
the  leaf.  In  some  cases,  as  in  Corn,  the  lack  of  water  causes  the 
leaves  to  roll,  in  which  case  there  is  not  a  good  exposure  to  light. 

For  the  most  active  photosynthesis  an  abundance  of  chloro- 
plasts  well  supplied  with  chlorophyll  is  also  necessary.  As  farmers 
know,  Corn  pale  in  color  does  not  grow  so  rapidly  as  Corn  that 
is  dark  green. 

Transpiration  from  Plants 

Transpiration  is  the  loss  of  water  in  the  form  of  vapor  from 
living  plants.  Transpiration,  although  similar  in  many  ways  to 
ordinary  evaporation,  differs  from  the  latter  process  in  that  it  is 
modified  by  the  structures  and  vital  activities  of  the  plant.  By 
transpiration  plants  are  almost  constantly  losing  water  to  the  air. 
It  is  for  this  reason  that  shoots  quickly  wilt  when  their  connec- 
tions with  roots  are  severed,  so  that  they  receive  no  water  from 
the  soil  to  compensate  for  the  loss  of  water  to  the  air.  The 
rapidity  with  which  green  grass  or  weeds  wilt  when  mowed  on  a 
hot  day  is  a  matter  of  common  observation.  Transpiration  is 
not  limited  to  leaves;  but  all  parts  of  plants  above  ground  are 
exposed  to  transpiration.  Fruits  and  seeds,  although  usually 
jacketed  in  a  rather  heavy  covering,  lose  water  during  storage. 
Even  during  winter,  the  buds,  twigs,  and  branches  of  trees  are 
continuously  losing  water  to  the  air.     However,  the  leaves,  on 


TRANSPIRATION   FROM   PLANTS  261 

account  of  many  openings  and  the  exposure  of  much  surface,  are 
the  regions  where  water  is  lost  most  rapidly.  Transpiration  is 
the  chief  enemy  of  plants  and  is  an  important  factor  in  determin- 
ing the  form,  structure,  and  distribution  of  plants. 

The  loss  of  water  is  not  so  much  under  the  plant's  control  as 
photosynthesis  and  respiration  are.  Unless  the  air  about  the 
plant  is  already  saturated  with  moisture  —  and  it  seldom  is  —  it 
will  take  up  water  wherever  water  is  available,  and  the  moist  tis- 
sues of  plants  are  available  sources  of  moisture.  The  air  circulat- 
ing through  the  inter-cellular  spaces  of  the  leaf  receives  moisture 
from  the  tissues,  and  consequently  its  moisture  content  becomes 
greater  than  that  of  the  air  outside  of  the  leaf.  But  according 
to  the  law  of  diffusion,  the  water-vapor  diffuses  from  the  air 
within  through  the  stomata  to  the  drier  air  without,  and  this 
diffusion  continues  as  long  as  the  air  within  the  leaf  receives 
sufficient  moisture  from  the  tissues  to  maintain  a  moisture  con- 
tent greater  than  that  of  the  air  without.  The  more  the  moisture 
content  of  the  air  within  and  without  differs,  the  more  rapidly 
the  plant  loses  water. 

Transpiration  can  be  easily  demonstrated  by  enclosing  a 
potted  plant  in  a  bell  jar,  taking  the  precaution  to  cover  all 
evaporating  surfaces,  except  the  plant,  with  rubber  cloth  or  wax. 
It  can  be  demonstrated  also  by  enclosing  a  branch  of  a  plant  in  a 
flask.  In  a  short  time  moisture  collects  on  the  glass,  at  first  as  a 
mist  which  may  later  form  into  drops  and  run  down  the  sides 
of  the  jar  or  flask,  {Fig.  237.)  This  indicates  that  the  plant 
loses  water  to  the  air,  which  consequently  becomes  so  nearly 
saturated  that  moisture  is  condensed  on  the  glass.  If  a  plant 
with  pot  protected  from  evaporation  is  exposed  to  transpiration 
and  weighed  at  intervals,  the  loss  in  weight  due  to  the  loss  of 
water  through  transpiration  is  quite  marked. 

The  amount  of  water  transpired,  although  varying  much  with 
conditions  and  in  different  plants,  is  always  a  large  proportion  of 
the  amount  absorbed.  Despite  the  fact  that  much  water  is 
used  by  the  plant  in  making  sugar  and  other  compounds  and  in 
maintaining  the  turgor  of  cells,  much  the  larger  proportion  of 
the  water  taken  in  by  the  roots  passes  through  the  plant  and  out 
into  the  air.  The  amount  of  water  transpired  under  various 
conditions  ranges  from  almost  zero  up  to  300  grams  or  more  per 
square  meter  of  leaf  area  per  hour.     For  this  unit  of  leaf  area  per 


262 


LEAVES 


hour,  transpiration  in  greenhouses  often  drops  to  10  grains  or  less 
at  night  and  rises  to  50  and  often  to  100  or  more  grams  during 
the  day.  For  plants  outside  where  there  is  more  exposure  to 
transpiration  the  variation  is  much  greater. 

As  compared  with  the  dry  weight  produced,  the  amount  of 
water  transpired  by  the  plant  is  surprising.     It  has  been  esti- 


FiG.  237.  —  Branch  of  a  plant  enclosed  in  a  flask  in  which  the  air  has 
become  so  moist  through  transpiration  from  the  enclosed  leaves  that  mois- 
tm-e  has  condensed  on  the  flask. 


mated  that  in  the  Central  United  States  about  425  pounds  of 
water  are  transpired  for  each  pound  of  dry  matter  produced  by 
the  plant.  It  is  stated  that  for  the  production  of  one  pound  of 
dry  matter.  Corn  requires  272,  Potatoes  423,  Red  Clover  453, 
and  Oats  557  pounds  of  water.  Calculated  on  the  same  basis, 
the  production  of  one  acre  of  Oats  of  average  yield  requires  945 
tons  of  water.  According  to  estimates,  an  Apple  tree  having 
thirty  years  of  growth  may  lose  on  an  average  of  250  pounds  of 
water  per  day,  or  possibly  18  tons  of  water  during  a  growing 
season.  An  orchard  of  40  such  trees  would  transpire  about  700 
tons  in  a  season.  It  has  been  estimated  that  even  an  acre  of 
Grass  may  transpire  from  500  to  700  tons  of  water  during  a 
season.     Now,  if  an  orchard  is  in  sod,  then  there  is  the  loss  of 


ADVANTAGES  OF  TRANSPIRATION  263 

water  from  both  trees  and  Grass.  These  estimates  show  the 
importance  of  maintaining  for  plants  a  suitable  supply  of  moisture 
in  the  soil. 

Conditions  Affecting  Transpiration.  —  The  humidity  of  the 
air,  temperature,  light,  and  velocity  of  wind  influence  trans- 
piration. 

The  Humidity  of  the  air  is  an  important  factor  in  transpira- 
tion. Other  conditions  remaining  constant,  transpiration,  in 
general,  increases  with  the  dryness  of  the  air.  For  this  reason 
hay  cures  quickly  when  the  atmosphere  is  dry.  It  is  also  during 
hot  days  when  the  air  is  dry  that  plants  are  most  likely  to  wilt. 

Since  heat  hastens  evaporation,  transpiration  usually  rises  with 
the  temperature  of  the  surrounding  air.  Also  light,  such  as  the 
bright  sunshine  that  is  common  on  hot  days,  is  an  important 
factor  in  raising  the  temperature  of  leaves,  which  thereby  have 
their  transpiration  increased.  In  bright  sunlight,  a  large  per 
cent  of  the  light  absorbed  by  leaves  is  changed  to  heat,  which  may 
raise  the  temperature  of  the  leaf  to  10°  or  15°  C.  higher  than  the 
temperature  of  the  surrounding  air;  and  this  surplus  of  heat 
induces  a  more  rapid  vaporization  of  the  water  within  the  leaf. 

The  velocity  of  the  wind  is  an  important  factor  in  transpira- 
tion; for  it  is  well  known  that  the  movement  of  the  air  has  an 
important  effect  on  the  rate  of  evaporation.  Thus  wind  moving 
30  miles  an  hour  evaporates  water  about  6  times  as  rapidly  as 
calm  air.  It  is  for  this  reason  that  muddy  roads  dry  more  rapidly 
on  windy  days.  When  the  air  is  calm,  the  air  about  the  plant 
becomes  more  nearly  saturated  and  consequently  ceases  to  take 
water  from  the  plant  so  rapidly;  but  when  the  air  is  dry  and 
rapidly  moving,  the  plant  is  constantly  enveloped  in  dry  air 
which  permits  very  little  diminution  in  the  rate  of  transpiration. 
When  winds  are  both  hot  and  dry,  they  are  very  destructive 
to  plants.  The  dry  hot  winds  of  some  of  the  Western  states 
sometimes  rob  plants  of  water  so  rapidly  that  crops  are  killed  in 
a  few  hours. 

Advantages  of  Transpiration.  —  Transpiration  is  an  advantage 
to  the  plant  in  two  ways.  First,  it  is  an  important  factor  in  main- 
taining the  flow  of  water  and  dissolved  substances  from  the  roots 
to  the  leaves  and  other  portions  of  the  shoot.  Second,  by  lower- 
ing the  temperature  of  plants,  it  often  prevents  injury  from 
excessive  heat. 


264  LEAVES 

As  the  student  well  knows,  the  movement  of  water  and  dis- 
solved substances  into  and  out  of  living  cells  is  in  accordance  with 
the  laws  that  govern  the  passage  of  liquids  through  membranes. 
But  in  passing  from  roots  to  leaves  and  other  parts  of  the  shoot, 
the  water  with  the  substances  in  solution  passes  through  the 
tube-like  xylem  vessels,  which  are  composed  of  the  cell  walls  of 
dead  cells,  and  in  such  cells,  with  cell  membrane  and  all  parts 
of  the  protoplasm  absent,  the  structural  features  upon  which 
osmosis  depends  are  not  present.  Of  course  throughout  the  stem 
and  roots  the  osmotic  activity  of  living  cells  around  the  xylem 
may  have  something  to  do  with  the  movement  of  liquids  through 
the  vessels,  but  this  force  combined  with  capillarity  and  root 
pressure  seems  entirely  inadequate  to  carry  water  from  the  roots 
to  the  tops  of  tall  trees.  That  transpiration  has  much  to  do  with 
the  movement  of  water  through  the  xylem  vessels  has  been  quite 
well  demonstrated  by  a  number  of  experiments. 

A  column  of  water,  due  to  the  coherence  of  the  water  mole- 
cules, holds  together  much  like  a  thread  or  rope.  The  coherence 
of  water  molecules  is  shown  by  the  way  water  drops  maintain 
themselves  when  hanging  on  the  end  of  a  pipette  or  on  the  eave 
of  a  building  where,  by  accumulating  and  freezing  while  still 
clinging,  they  form  icicles.  It  has  been  demonstrated  that  even 
very  small  columns  of  water,  like  those  reaching  from  roots  to 
the  leaves  through  the  xylem  vessels,  are  able  to  endure  heavy 
strains  without  breaking.  Regarding  the  columns  of  water 
through  the  vessels  as  small  but  tough  threads  with  one  end  in 
contact  with  the  soil  water  at  the  roots  and  the  other  end  in 
contact  with  the  cell  sap  in  the  mesophyll  cells  of  the  leaf,  it  is 
evident  that  whenever  water  becomes  scarce  in  the  mesophyll 
cells  through  transpiration,  then  by  osmosis  these  columns  of 
water  will  be  pulled  in  until  the  cells  of  the  mesophyll  are  so  filled 
with  water  and  their  cell  sap  so  diluted  that  they  no  longer  have 
the  osmotic  force  to  overcome  the  resistance  of  the  water  columns. 
But  since  transpiration  is  practically  continuous,  although  varying 
much  in  rate  at  different  times,  the  water  columns  are  drawn  into 
the  cells  of  the  mesophyll  almost  continuously,  and  hence  the 
apparently  continuous  flow  of  water  and  dissolved  substances 
i  hrough  the  xylem  of  plants.  Thus,  transpiration,  by  removing  the 
water  from  the  cells  of  the  leaf  and  thereby  causing  the  dissolved 
substances  in  the  sap  of  these  cells  to  become  more  concentrated, 


DANGERS   RESULTING   FROM   TRANSPIRATION  265 

brings  about  the  osmotic  force  by  which  the  cells  of  the  leaf  draw 
in  the  water  columns.  The  energy  contributed  by  transpiration  is 
really  the  heat-energy  involved  in  changing  water  into  vapor,  in 
which  form  the  water  escapes  from  the  plant.  Such  seems  to  be 
the  relation  of  transpiration  to  the  ascent  of  sap,  but  what  other 
factors  are  involved  and  to  what  extent  we  have  no  definite  knowl- 
edge, and,  therefore,  we  may  attribute  too  much  to  transpiration. 

It  was  once  generally  believed  that  the  flow  of  water  through 
the  plant  is  necessary  to  transport  the  mineral  elements  of  the 
soil  to  the  different  regions  of  the  shoot,  and  that  the  amount  of 
the  mineral  elements  reaching  the  leaves  and  other  parts  of  the 
shoot  is  directly  related  to  the  amount  of  water  flowing  through 
the  plant  and,  therefore,  to  transpiration.  But  some  experiments 
indicate  that  in  some  cases,  at  least,  the  process  of  diffusion  by 
which  the  mineral  elements  and  other  substances  in  solution  pass 
to  those  regions  where  they  are  less  concentrated,  regardless  of 
the  movement  of  the  water  in  which  they  are  dissolved,  can  supply 
the  mineral  elements  to  different  parts  of  the  shoot  as  rapidly  as 
needed.  In  fact,  in  case  of  Tobacco  plants,  analyses  have  shown 
that  plants  grown  in  the  shade  may  have  a  higher  mineral  content 
than  plants  grown  exposed  to  excessive  transpiration.  In  other 
words,  the  plants  through  which  the  least  water  flows  may  take  the 
most  mineral  from  the  soil.  However,  since  the  water  carries  the 
dissolved  substances  along  in  its  current,  the  movement  of  water 
through  the  plant  tends  to  aid  diffusion  in  the  distribution  of  the 
elements  in  solution. 

Since  transpiration,  like  evaporation,  is  a  cooling  process,  it 
often  prevents  leaves  from  becoming  overheated.  Sometimes 
bright  sunshine,  following  a  summer  shower  which  has  filled  the 
air  with  moisture,  results  in  the  leaf  injury  known  as  scalding. 
Under  these  conditions,  transpiration  is  checked  and  the  tempera- 
ture of  the  leaf  becomes  too  high.  As  a  large  part  of  the  sunlight 
is  changed  into  heat  by  the  leaf,  the  heat  accumulates  very 
rapidly  in  bright  sunshine.  It  has  been  found  in  the  case  of 
some  leaves  that  the  excess  of  heat,  if  transpiration  be  stopped, 
may  raise  the  internal  temperature  of  the  leaf  to  the  death  point 
in  a  few  minutes.  Transpiration,  therefore,  rids  the  leaf  of  the 
dangerous  excess  of  heat. 

Dangers  Resulting  from  Transpiration.  —  So  long  as  water 
from  the  roots  can  be  supplied  as  rapidly  as  water  is  lost  by 


266 


LEA\^S 


transpiration,  the  plant  is  not  in  danger.  But  it  is  not  uncommon 
to  see  Corn  with  leaves  rolled  and  Potatoes,  Cotton,  Clover,  and 
other  plants  wilted  during  dry  hot  days.  These  plants  are  losing 
water  faster  than  it  can  be  replaced  from  the  roots.  These 
plants  are  in  danger  because  their  living  cells  are  becoming  dry, 
and  too  much  drying  results  in  death.  More  plants  die  on 
account  of  transpiration  than  anything  else. 

The  important  thing  for  the  plant  is  the  maintenance  of  a 
proper  balance  between  supply  and  loss  of  water.  The  plant  can 
endure  rapid  transpiration,  if  a  copious  supply  of  water  is  coming 
up  from  the  roots;  but,  if  the  ground  is  dry  about  the  roots,  the 
root  system  small,  or  water  hard  to  obtain  from  the  soil,  as  is  the 


Fig.  238.  —  A  portion  of  a  cross 
section  through  a  node  of  Sugar 
Cane,  showing  rods  of  wax  secreted 
by  the  epidermis.  Enlarged  many 
times.     After  De  Bary. 


TciaooaciQOO.i 


^lfc> 


Fig. 


239.  —  A  portion   of  a 


tion  through  a  Mullein  leaf,  show- 
ing the  epidermis  with  its  branched 
hairs.     After  Andrews. 


case  in  soils  that  are  cold  or  frozen,  then  even  a  small  amount  of 
transpiration  may  be  injurious. 

Protection  against  Injuries  Resulting  from  Transpiration.  — 
Plants  may  be  protected  against  the  injurious  effects  of  trans- 
piration by  having  their  transpiring  surface  modified,  or  by 
having  the  soil  moisture  increased  or  conserved. 

There  are  various  ways  in  which  plants  modify  their  transpiring 
surface.  Some  plants,  such  as  the  Carnation,  Pine,  and  many 
plants  of  the  desert,  have  the  epidermis  of  their  leaves  covered 
with  a  heavy  layer  of  cutin.  Sometimes,  as  in  Cabbage,  Sugar 
Cane,  and  Wheat,  the  epidermis  is  covered  with  a  waxy  bloom. 
(Fig.  238.)  Many  plants  are  protected  by  a  covering  of  hairs. 
{Fig.  239.)  Some  plants,  such  as  the  Cacti  of  the  desert,  have 
reduced  their  leaves  to  mere  spines  which  offer  only  little  trans- 


PROTECTION   AGAINST  TRANSPIRATION 


267 


piring  surface.  {Fig.  240.)  By  reducing  the  number  of  stoniata, 
as  in  many  Grasses,  or  by  sinking  the  stomata  in  special  epider- 
mal -cavities,   as  in   the   Carnation,    transpiration   is   reduced. 


Fig.  240.  —  A  globular  cactus,  an  example  of  a  plant  having  leaves 
replaced  by  spines.     After  J.  M.  Coulter. 

Sometimes,  as  in  the  Corn,  the  rolling  of  the  leaves  decreases 
the  surface  exposed  and  lessens  transpiration.     {Fig.  2^1  •)     The 


Fig.  241.  —  Cross  section  of  a  Com  leaf.  I,  lower  epidermis;  ;/,  upper 
epidermis.  Notice  that  the  cells  are  larger  on  the  upper  side  than  on  the 
lower  side  of  the  leaf.  The  cells  of  the  upper  epidermis,  being  larger,  shrink 
more  than  those  of  the  lower  epidermis,  and  thus  cause  the  rolling  of  the 
leaf  in  dry  weather.     Much  enlarged. 

leaves  may  have  an  edgewise  position  and  thereby  avoid  the 
direct  rays  of  the  midday  sun,  as  Wild  Lettuce  illustrates. 

The  shedding  of  leaves  from  the  plant  is  an  important  means  of 
protection.     Many  of  our  trees  shed  some  of  their  leaves  during  a 


268  LEAVES 

summer  drought  and  thereby  decrease  their  transpiring  surface. 
Of  course  this  is  not  a  protection  to  the  leaves  but  to  the  plant. 
Most  trees  of  the  temperate  region  shed  all  their  leaves  in  autumn. 
Such  trees  are  known  as  deciduous.  This  shedding  of  leaves  in 
autumn  protects  the  plant  against  transpiration  during  winter. 
Even  with  leaves  absent,  trees  are  sometimes  killed  by  trans- 
piration from  buds  and  twigs.  The  killing  by  transpiration  in 
winter  is  not  due  to  a  great  water  loss,  but  to  the  inability  of  the 
roots  to  furnish  water  to  compensate  for  the  loss.  Since  the  roots 
of  most  trees  are  not  far  below  the  surface,  a  deep  freeze  may 
freeze  the  water  about  them.  Even  when  the  soil  is  cold,  roots 
take  up  water  slowly,  and  when  the  water  is  frozen  into  ice,  they 
can  not  absorb  it  at  all.  With  only  a  little  water  furnished  by 
the  roots,  a  small  amount  of  transpiration  may  be  sufficient  to 
cause  the  death  of  the  cells  in  the  buds  and  twigs. 

In  transplanting  trees,  it  is  usually  necessary  to  prune  the  top, 
because  the  root  system  has  been  partly  broken  and  cut  away, 
and  consequently  is  not  able  to  furnish  enough  water  to  compen- 
sate for  the  amount  transpired  from  a  shoot  of  normal  size. 
Pruning  the  top  results  in  fewer  leaves  and  hence  less  transpiring 
surface.  Even  after  trees  have  been  transplanted  and  well 
established,  a  reduction  of  the  transpiring  surface  by  pruning  the 
top  is  often  helpful,  but  usually  the  pruning  of  such  trees  has 
other  purposes  as  pointed  out  in  the  study  of  buds.  However, 
since  the  leaves  are  food-making  organs,  only  a  limited  number 
of  them  can  be  removed  or  the  plant  will  suffer  from  starvation. 

Supplying  moisture  to  the  soil  protects  against  injuries  result- 
ing from  transpiration.  Plants  in  the  greenhouse  must  have  the 
soil  about  their  roots  kept  moist  by  watering.  In  the  dry 
western  regions  water  is  supplied  to  the  soil  by  methods  of 
irrigation. 

Much  can  be  done  in  protecting  against  transpiration  by 
conserving  the  moisture  of  the  soil.  If  an  orchard  is  in  sod, 
many  tons  of  water  will  be  lost  from  the  soil  through  the  trans- 
piration of  the  grass.  By  plowing  and  keeping  the  ground  free 
from  grass  and  weeds,  the  water  of  the  soil  is  conserved  for  the 
fruit  trees.  In  regions  where  dry  farming  is  practiced,  the 
ground  is  fallowed  during  one  year  and  then  seeded  the  second 
year.  Fallowing  consists  in  keeping  the  ground  plowed  and  well 
harrowed,  so  that  the  surface  will  be  covered  with  a  mulch  and 


RESPIRATION  269 

be  free  from  vegetation.  This  treatment  allows  the  water  from 
snows  and  rains  to  soak  into  the  soil  readily  and  also  prevents 
much  loss  of  water  through  evaporation.  This  stored-up  water 
is  then  used  by  the  crop  during  the  second  year. 

Many  plants  which  live  in  dry  regions  have  regions  of  water 
storage.  For  example,  the 
Cacti  store  much  water  in 
their  stems  and  this  storage 
enables  them  to  withstand 
very  dry  periods.  Some 
plants,  like  the  Begonia,  have 
special  cells  in  their  leaves 
for  the  storage  of  water. 
{Fig.  242.)  In  many  plants, 
as  in  Corn,  much  water  is 
stored  in  the  pith. 

Thus  it  is  seen  that  trans- 
piration is  helpful  when  the 

i        1     ,      1  ,  ,       Fig.  242. — A  small  portion  of  a  cross 

water    lost    does    not    exceed       ^.        ,     ^        •    ,    <■    1      • 

section  of  a  Begonia  leaf,  showing  water 
the     supply     furnished     from    storage  cells  (.s)  and  chlorenchyma  (/). 
the  roots;  also  that  the  rate 

of  transpiration  depends  much  upon  temperature,  humidity  of 
the  air,  light,  and  velocity  of  the  wind;  and  that  the  dangers  of 
transpiration  may  be  overcome  by  modifying  the  transpiring 
surface  or  by  maintaining  an  adequate  supply  of  water  in  the 
soil  or  in  storage  regions  of  the  plant. 

Respiration 

Although  respiration  is  a  fundamental  process  in  all  living 
cells,  leaves  afford  a  good  place  for  observing  the  outward  signs 
of  it.  Most  of  the  oxygen  used  in  the  respiration  of  the  plant 
enters  at  the  leaves  from  which  place  it  is  carried  to  all  parts  of 
the  plant.  Also  through  the  leaves  much  of  the  carbon  dioxide 
produced  by  respiration  escapes  to  the  air.  Respiration  and 
photosynthesis,  although  occurring  together  in  leaves,  are  wholly 
separate  processes  as  shown  by  the  ways  in  which  they  differ. 
First,  photosynthesis  occurs  in  chloroplasts  and  is  a  synthetic  proc- 
ess, in  which  the  elements  of  carbon  dioxide  and  water  are  built 
into  compounds  with  the  storage  of  latent  energy,  while  respira- 
tion occurs  in  all  parts  of  the  protoplasms  and  is  a  process  in  which 


270  LEAVES 

compounds  are  broken  into  their  constituents,  usually  through 
oxidation,  with  the  result  that  the  latent  energy  is  released  to  be 
used  in  various  kinds  of  work.  Second,  photosynthesis  uses 
carbon  dioxide  and  releases  oxygen,  while  respiration  uses  oxygen 
and  releases  carbon  dioxide.  Hence  one  liberates  the  gas  which 
the  other  uses,  and  in  this  way  the  two  processes  tend  to  sup- 
port each  other.  When  both  processes  are  active  at  the  same 
time,  as  during  the  day,  each  process  tends  to  obscure  the  other 
by  using  the  gases  liberated  before  these  gases  escape  from  the 
leaf.  However,  when  photosynthesis  is  active,  the  amount  of 
carbon  dioxide  used  and  oxygen  liberated  is  so  much  greater  than 
the  gaseous  exchanges  of  respiration  that  the  latter  process  is 
entirely  obscured.  On  this  account,  botanists  once  thought  that 
respiration  was  a  process  performed  only  by  animals  and  that  the 
plant  breathes  in  a  way  just  opposite  from  that  of  animals.  Of 
course  further  investigations  showed  that  plants  respire  just  the 
same  as  animals  do,  but  in  addition  green  plants  carry  on  photo- 
synthesis which,  when  active,  so  much  obscures  respiration  that 
the  latter  process  had  escaped  notice.  Third,  photosynthesis 
depends  upon  the  presence  of  light,  while  respiration  is  inde- 
pendent of  light,  being  active  at  night  as  well  as  in  the  daytime. 
At  night  when  there  is  no  photosynthesis  to  obscure  respiration, 
plants  take  in  oxygen  and  liberate  carbon  dioxide  just  as  animals 
do,  and  the  notion  once  prevalent  that  plants  purify  the  air  is 
only  true  of  them  when  they  are  engaged  in  photosynthesis. 

Respiration  and  transpiration,  although  influencing  each  other 
to  some  extent,  are  also  distinct  processes.  Since  respiration 
liberates  energy,  some  of  which  is  in  the  form  of  heat,  respiration 
may  increase  transpiration  by  raising  the  temperature  of  the  leaf. 
Furthermore,  respiration  in  breaking  down  compounds  releases 
water  in  the  form  of  vapor,  in  which  form  it  readily  escapes  to  the 
air.  On  the  other  hand,  when  transpiration  reduces  the  water 
content  of  cells  so  much  as  to  interfere  with  the  activities  of  the 
protoplasm,  then  respiration  may  be  retarded. 


Special  Forms  of  Leaves 

In  contrast  to  the  leaves  which  we  have  been  studying,  there 
are  some  leaves  which  have  become  so  modified  as  to  resemble 
ordinary  leaves  very  little.     Some  have  become  so  changed  that 


SPECIAL   FORMS  OV  LEAVES 


271 


Fig.  243. 


A  portion  of  a  Sweet  Pea,  showing  one  leaf  (l),  a  portion 
of  which  is  transformed  into  a  tendril  (/). 


they  have  lost  much  or  all  of  their  power  to  make  food,  and  have 
become  apparently  useless  or  have  taken  on  other  functions. 

A  very  common  modified  form  of  the 
leaf  is  the  scale.  The  most  familiar 
example  of  scales  is  furnished  by  the 
buds  of  shrubs  and  trees,  where  they 
form  a  protection  for  the  inner  vital 
portions  of  the  bud.  These  scales  are 
considered  leaves  which  have  been  pre- 
vented from  developing  by  being  so 
closely  crowded  in  the  overlapping  ar- 
rangement. The  leaves  of  underground 
stems,  which  do  not  get  to  the  light, 
appear  as  small  scale-like  bodies  with- 
out green  tissue,  and  apparently  have 
no  function.  Sometimes  scales  are 
fleshy  and  are  used  for  food  storage,  as 
in  Lily  bulbs.  Onions,  etc.  In  the  Asparagus  the  leaves  are 
scale-like  and  the  food-making  is  mostly  done  by  the  stem. 


Fig.  244.  —  A  branch  of  a 
Barberry,  showing  the  leaves 
transformed  into  thorns. 


272 


LEAVES 


Leaves  may  sometimes  develop  into  tendrils,  either  the  entire 
leaf  or  only  a  part  of  it  becoming  tendrils,  as  in  the  Sweet  Pea. 
(Fig.  243.) 

In  the  Barberrv  and  some  othei-  shrubs,  the  leaves  are  so  modi- 


FiG.  245.  —  Pitcher  Plant,  showing  pitcher-Uke  leaves  (I). 


Fig.  246.  —  Sundew,  showing  the  leaves  which  catch  and  digest  insects. 


tied  as  to  form  thorns.  {Fig.  244-)  Sometimes,  as  in  the  Com- 
mon Locust,  only  a  portion  of  the  leaf  is  devoted  to  the  formation 
of  thorns. 

The  most  interesting  special  forms  of  leaves  are  those  adapted 


USES  OF   THE   PHOTOSYNTHEIIC   FOOD 


273 


to  catching  insects.  Phints  with  such  leaves  are  often  called 
"carnivorous  plants  "  or  "insectivorous  plants."  The  "Pitcher 
Plants  "  are  so  named  because  the  leaves  form  tubes  or  urns  of 
various  forms,  which  contain  water,  and  to  these  pitchers  insects 
are  attracted  and  then  drowned.  (Fig.  245.)  The  plants  known 
as  "  Sundews "  have  their  leaves  spread  on  the  ground  and 
clothed  with  secreting  hairs. 
(Fig,  246.)  These  secre- 
tions not  only  entangle  in- 
sects but  digest  them.  In 
the  "Venus  Flytrap,"  por- 
tions of  the  leaves  work  like 
steel  traps  and  hold  the  in- 
sects fast  until  digested. 
(Fig.  247.) 

Uses  of  the  Photosynthetic 
Food 

In  plants,  as  in  animals, 
the  chemical  processes  upon 
which  growth  and  other  vital 
activities  depend  are  both 
constructive  and  destruc- 
tive. While  the  simpler  ele- 
ments are  being  transformed 
into  complex  substances, 
complex  substances  through 
respiration  and  other  destructive  processes  are  being  broken  into 
simpler  substances.  These  chemical  transformations  constitute 
metabolism,  and  are  said  to  be  anabolic  when  constructive  and 
catabolic  when  destructive.  Through  the  plant's  metabolic 
processes  numerous  substances  are  formed,  of  which  protoplasm, 
proteins,  sugars,  starches,  fats,  oils,  hemicellulose,  amino-com- 
pounds,  cellulose,  wood,  cutin,  suberin,  enzymes,  acids,  tannins, 
glucosides,  and  alkaloids  are  common  ones.  All  of  these  sub- 
stances are  thought  to  be  of  some  use  to  the  plant,  although  the 
exact  function  of  some  of  them  is  not  definitely  known. 

The  photosynthetic  grape  sugar,  since  there  is  much  evidence 
that  in  most  cases  it  is  the  chief  food  formed  by  photosynthesis, 
may  be  regarded  as  a  foundational  food;   for  the  photosynthetic 


Fig.  247.  —Venus  Flytrap,  showing 
the  leaves  which  open  and  close  in  catch- 
ing insect? 


274  LEAVES 

sugar  furnishes  either  all  or  an  important  part  of  the  constituents 
of  which  all  other  plant  substances  are  composed.  Even  if  light 
is  essential  to  the  formation  of  proteins,  which  are  formed  so 
abundantly  in  leaves  and  carried  away  like  the  photosynthctic 
sugar  to  other  parts  of  the  plant  for  use,  sugar  is  an  important 
constituent  of  proteins  and  is  therefore  a  basal  substance  in  their 
formation.  Since  animals  obtain  their  food  either  directly  or 
indirectly  from  plants,  it  is  evident  that  the  photosynthetic  sugar 
is  also  the  basal  food  for  animals. 

The  various  metabolic  changes  in  the  plant  have  to  do  with 
providing  living  protoplasm,  a  frame  work,  reserve  foods,  secre- 
tions, and  energy. 

Protoplasm.  —  It  is  in  connection  with  protoplasm,  the  living 
substance  of  both  plants  and  animals,  that  the  metabolic  processes 
occur.  Protoplasm  not  only  transforms  substances  enclosed 
within  it  but  by  means  of  enzymes  which  it  secretes  it  is  also  able 
to  act  on  substances  with  which  it  is  not  in  contact.  Within  the 
protoplasm  sugar  is  synthesized  by  the  chloroplasts,  and  starch, 
proteins,  fats,  and  many  other  plant  products  are  constructed. 
At  the  same  time  substances  are  being  constructed  in  the  proto- 
plasm, substances  are  also  being  decomposed,  so  that  within  the 
protoplasm  substances  resulting  from  both  constructive  and 
destructive  processes  are  always  present.  So  it  is  not  at  all 
strange  that  many  kinds  of  substances  are  present  in  the  proto- 
plasm. Some,  like  starch  and  some  proteins,  are  insoluble,  while 
many,  like  the  sugars  and  acids,  are  dissolved  in  the  nuclear  and 
cell  sap.  That  protoplasm  can  transform  substances  with  which 
it  is  not  in  contact  is  well  illustrated  in  seeds,  where  enzymes 
secreted  by  the  embryo  diffuse  out  to  the  endosperm  and  trans- 
form it  into  soluble  forms  of  food. 

One  of  the  important  constructive  processes  of  protoplasm  is  the 
formation  of  protoplasm.  As  the  plant  grows  and  more  cells  are 
formed,  more  of  the  elements  of  chromatin,  nucleoli,  cytoplasm, 
and  all  other  protoplasmic  structures  must  be  formed. 

As  to  the  chemical  composition  of  protoplasm  in  its  living 
state,  we  have  no  definite  knowledge.  Chemical  analyses  of 
dead  protoplasm  show  that  a  large  number  of  substances  are 
present,  of  which  proteins  are  the  largest  and  most  essential  part. 
The  different  kinds  of  proteins  vary  in  composition,  but  all  are 
composed  chiefly  of  carbon,  hydrogen,  oxygen,  and  nitrogen.     In 


FRAMEWORK  OF  THE  PLANT 


275 


addition  to  these  elements,  most  proteins  contain  a  small  amount 
of  sulphur  and  some  proteins  also  contain  a  small  amount  of 
phosphorus.  By  chemically  combining  in  certain  proportions 
the  carbon,  hydrogen,  and  oxygen  of  the  sugar  with  the  nitrogen, 
sulphur,  and  phosphorus  obtained  from  the  soil,  proteins  are 
formed,  but  as  to  how  the  proteins  are  transformed  into  living 
protoplasm  no  one  knows. 

Framework  of  the  Plant.  —  Protoplasm,  since  it  is  a  semi- 
fluid, has  no  definite  shape  except  when  enclosed  in  a  framework. 
It  is  by  means  of  a  frame- 
work that  higher  plants  are 
able  to  so  shape  themselves 
as  to  be  adjusted  to  the  soil, 
air,  and  sunlight. 

The  cell  walls  constitute 
the  framework.  They  are 
so  joined  as  to  divide  the 
plant  into  the  compart- 
ments in  which  the  pro- 
toplasts or  individual 
masses  of  protoplasm  re- 
side. The  fact  is,  how- 
ever, that  each  cell  is  en- 
closed by  walls  of  its  own  ; 
but  the  adjacent  walls  of 
neighboring  cells  are  usu- 
ally so  closely  joined  that 
the  cells  appear  to  be  sep- 
arated by  a  single  wall. 
Through  very  small  pores  in  the  cell  walls,  the  protoplasts  are 
commonly  connected  by  small  protoplasmic  strands,  which  afford 
a  means  of  communication  between  the  protoplasts  of  neighbor- 
ing cells.     {Fig.  248.) 

The  primary  substance  of  which  cell  walls  are  formed  is 
cellulose,  a  substance  closely  related  to  sugar  as  its  formula 
(C6Hio05)n  indicates.  In  the  formula  (C6Hi()()5)n  each  combina- 
tion CeHioOb,  of  which  an  unknown  (n)  nmnber  are  combined  in 
forming  cellulose,  is  a  molecule  of  sugar  minus  a  molecule  of  water 
as  may  be  seen  from  the  equation  C6H12O6  —  H2O  =  CeHioOs. 
Thus  the  formation  of  cellulose  involves  no  other  elements  than 


Fig.  248.— Cells  with  protoplasm  shrunken, 
so  that  the  fine  strands  of  protoplasm  extend- 
ing through  the  cell  walls  and  connecting 
neighboring  protoplasts  may  be  seen.  Highly 
magnified. 


276  LEAVES 

those  of  grape  sugar  and  only  slight  changes  in  their  pro- 
portion. 

Cellulose  is  a  suitable  material  for  cell  walls;  for  its  elasticity 
permits  cells  to  enlarge,  and  its  permeability  allows  water 
and  solutions  of  food  to  reach  the  protoplasm.  In  actively 
growing  cells  thin  cellulose  walls  are  essential,  so  that  the 
cells  can  enlarge;  but  when  the  cells  are  to  afford  strength, 
as  in  case  of  bast  fibers,  then  the  cellulose  is  so  deposited  as  to 
form  thick  walls.  Commonly  when  an  important  function  of 
the  cell  walls  is  to  afford  strength,  another  substance  called 
lignin  is  formed  from  the  sugar  and  combined  with  the  cellulose, 
thus  forming  the  wood  characteristic  of  the  trunks  of  trees  and 
shrubs  but  also  common  in  herbaceous  plants.  Also  in  the  shells 
of  some  nuts  and  the  coats  of  many  seeds,  lignified  walls  are 
common.  Woody  walls,  like  cellulose  walls,  are  permeable  to 
water  and  solutions,  and  for  this  reason  are  not  adapted  for 
protective  coverings,  where  the  prevention  of  loss  of  water  is  an 
important  function.  In  forming  waterproof  walls,  a  portion 
of  the  sugar  or  cellulose  is  converted  into  fatty  or  wax-like 
substances  known  as  cutin  and  suherin.  Cutin  is  common  in  the 
outer  walls  of  epidermal  cells,  while  suberin  occurs  throughout 
the  walls  of  cork. 

Occasionally  in  the  formation  of  cell  walls,  some  of  the  sugar 
is  converted  into  substances  which  swell  and  become  mucilaginous 
when  wet,  as  the  seed  coats  of  Flax  and  some  Mustards  illustrate. 

Some  other  substances  which  are  formed  from  sugar  and  asso- 
ciated with  cellulose,  lignin,  cutin,  and  suberin  in  cell  walls  are  the 
pectic  compounds.  The  pectic  substances  (pectin,  pectose,  and 
pectic  acids),  although  much  like  cellulose,  are  more  easily  decom- 
posed by  certain  acids  and  alkalies.  They  are  often  combined  with 
minerals,  and  one  of  the  mineral  compounds,  known  as  caldum 
pedate,  is  the  chief  substance  of  the  middle  portion  (middle 
lamella)  or  oldest  portion  of  walls  separating  cells. 

Besides  the  various  substances  formed  from  sugar,  cell  walls 
are  often  infiltrated  with  mineral  matters,  notably  silica,  and 
these  minerals  often  add  much  strength  to  the  frame  work. 

Cellulose  and  its  closely  allied  compounds  serve  man  in  many 
ways.  From  cellulose  paper  is  made,  and  long  cellulose  fibers,  as 
those  of  Cotton  and  Flax,  are  woven  into  clothing.  The  wood 
of  plants  is  the  source  of  lumber.      Being   oxidizable,   cellulose 


RESERVE   I'OODS  277 

and  its  compounds  serve  as  fuel,  either  in  the  form  of  wood,  peat, 
or  coal.  Man  converts  cellulose  into  celluloid,  artificial  silks, 
artificial  rubl)er,  and  powerful  explosives,  such  as  gun  cotton. 

Reserve  Foods.  — ■  The  photosynthetic  sugar,  which  is  not  used 
immediately  as  food,  is  stored  in  seeds,  stems,  roots,  and  tem- 
porarily in  leaves  for  use  at  some  future  time.  The  reserve  foods 
into  which  the  excess  of  photosynthetic  sugar  is  transformed  are 
of  various  forms,  but  are  chiefly  of  three  general  classes  —  carbo- 
hydrates, fats,  and  proteins. 

The  carbohydrates  include  the  sugars,  starches,  and  hemi- 
celluloses,  and  are  so  named  because  their  proportion  of  hydrogen 
and  oxygen,  being  the  same  as  in  water,  suggested  that  they  were 
compounds  of  carbon  and  water. 

Sugars  are  of  various  kinds,  but  only  a  few  occur  in  considerable 
quantities  in  the  plant.  Grape  sugar,  fruit  sugar,  and  cane  sugar, 
the  most  important  of  the  sugars,  are  commonly  present  in  the 
sap  of  plants.  Grape  sugar,  called  glucose  or  dextrose,  and  fruit 
sugar,  called  fructose  or  levulose,  are  the  simplest  of  the  sugars. 
They  have  the  same  formula  C6H12O6,  but  differ  in  the  arrange- 
ment of  atoms.  Both  are  found  in  all  parts  of  plants,  but  usually 
one  is  more  abundant  than  the  other.  In  sweet  fruits  and  the 
nectar  of  flowers,  fruit  sugar  is  usually  more  abundant  than  either 
glucose  or  cane  sugar,  while  in  Sugar  Cane,  where  both  occur 
along  with  cane  sugar,  glucose  is  more  abundant  than  fructose. 
Much  glucose  accumulates  in  the  stems  of  Corn  and  other  Grasses. 
Both  glucose  and  fructose  are  produced  not  only  synthetically, 
l)ut  also  through  the  decomposition  of  some  of  the  more  complex 
carbohydrates.  Thus  when  Cane  sugar  is  boiled  with  hydro- 
chloric acid,  glucose  and  fructose  in  equal  amounts  are  produced. 

Cane  sugar,  called  sucrose  or  saccharose,  is  the  sugar  of  most 
service  to  man.  It  is  present  in  the  sap  of  most  plants  and 
accumulates  in  great  abundance  in  Sugar  Cane,  Sorghum,  Beets, 
and  the  Sugar  Maple.  From  the  stems  of  Sugar  Cane  and  the 
roots  of  Sugar  Beets  many  million  tons  of  cane  sugar  are  extracted 
each  year.  A  molecule  of  cane  sugar,  as  represented  by  the 
formula  C12H22O11,  contains  a  molecule  of  glucose  and  one  of 
fructose  with  a  molecule  of  water  dropped  in  making  the  com- 
])ination.  The  formation  of  cane  sugar  is  represented  by  the 
equation  CeR^O,  +  CeHioOe  -  HoO  =  CioHooO„. 

Another  sugar,  known  as  Maltose  and  having  the  same  formula 


278  LEAVES 

as  cane  sugar  but  differing  in  arrangement  of  atoms,  occurs  in 
germinating  seeds,  and  is  especially  abundant  in  germinating 
Barley.  It  is  formed  when  starch  is  broken  into  its  components. 
Maltose,  when  broken  into  its  constituents,  gives  rise  to  two 
molecules  of  glucose. 

The  starches  are  the  most  abundant  storage  forms  of  food  into 
which  the  photosynthetic  sugar  is  converted.  They  occur  in  all 
parts  of  the  plant,  but  are  especially  abundant  in  seeds,  tubers, 
and  fleshy  roots,  where  they  are  extensively  used  by  man  for  food. 
The  food  value  of  Corn,  Wheat,  Rice,  and  other  Cereals,  and  of 
Potatoes  depends  mainly  upon  the  starch  which  they  contain. 
The  starches,  unlike  the  sugars,  are  in- 
soluble and  occur  in  the  form  of  definitely 
shaped  bodies,  known  as  starch  grains, 
which  vary  much  in  shape,  size,  and  mark- 
ings in  different  plants.  {Fig.  249.)  The 
size,  shape,  and  markings  of  the  starch 
grains  are  so  characteristic  of  many  plants 
that  by  a  study  of  the  starch  grains  in  a 
mixture  of  ground  vegetable  products,  the 
constituents  can  often  be  determined  and 
,  -  the  adulterants  thereby  detected. 

Fig  249  —a  starch  Starch  and  cellulose  have  the  same  for- 
graiu   of   Irish   Potato;  mula  (C6Hio05)n  but  they  differ  in  the  num- 

b,  starch  grain  of  Wheat;  ber  of  combinations,  CeHioOs,  contained  in 

c,  starch  grain  of  Corn.  ^j^^-^.  j^^^iecules.  Their  exact  difference  in 
structure  is  not  known,  for  the  number  of  combinations,  CeHioOs, 
contained  in  a  molecule  of  either  starch  or  cellulose  has  not  been 
determined.  They  differ  in  physical  properties  as  well  as  chemi- 
cally. The  starches  are  readily  converted  back  into  sugar,  in 
which  form  they  are  used  by  the  plant.  The  starches  are  broken 
into  simpler  compounds  by  a  group  of  enzymes,  and  the  prod- 
ucts formed  are  dextrin,  maltose,  and  glucose  according  to  the 
extent  to  which  the  starch  molecules  are  broken  up. 

The  hemi-celluloses  are  quite  prominent  in  some  seeds,  and 
occur  in  many  other  places  in  the  plant.  They  are  much  like 
ordinary  cellulose,  but,  being  easily  converted  into  sugar,  they 
are  available  sources  of  food  and  on  this  account  are  often  called 
reserve  celluloses.  They  are  usually  very  hard  substances  and 
are  deposited  as  extra  layers  on  the  cell  walls.     The  hardness  of 


RESERVE  FOODS 


279 


Date  seeds  and  Ivory  nuts  is  due  to  the  henii-celluloses,  in  which 
form  these  seeds  store  their  reserve  food.     {Fig.  250.) 

The  Jats  are  prominent  storage  forms  of  food  in  a  number  of 
plants  and  usually  occur  in  the  form  of  oils,  known  as  fatty  oils. 
They  contain  the  same  elements  as  the  carbohydrates,  but  have 
much  less  oxygen  in  proportion  to  carbon,  and  the  hydrogen  and 
oxygen  are  not  present  in  the  proportion  to  form  water.  The 
vegetable  fats  are  chiefly  compounds  of 
glycerine  and  fatty  acids.  Palmitic, 
oleic,  and  stearic  acids  are  the  fatty 
acids  commonly  found  in  fats,  but  there 
are  many  others,  and  the  character  cf  the 
fat  formed  depends  much  upon  the  kind 
of  fatty  acid  entering  into  the  combina- 
tion. The  fatty  oils  are  usually  stored 
in  the  seeds,  but  often  in  the  flesh  of 
fruits  and  other  portions  of  the  plant. 
Olive  oil  which  is  pressed  out  of  the  fruit 
of  the  Olive,  Corn  oil  from  the  embrj^os 
of  Corn,  Cotton-seed  oil  from  Cotton  seed, 
Coconut  oil  from  Coconuts,  Linseed  oil 
from    Flax    seed,    and    Castor   oil  from 

seeds  of  the  Castor  Bean  are  well  known  J^^'^  "^^  ^^^.°^"^s,^^,^  ^""r 

,       .      .,  ,  ,  .  ,       i      1  treme  thickening  with  hemi- 

plant  oils  and  have  an  important  place  ceUulose,  which  is  deposited 

in  our  industries.  in  layers  forming  striations. 

The  fatty  oils  are  usually  present  in  The  walls  are  perforated  by 
the  storage  cells  in  the  form  of  globules,  ^^^  ^^"^^^^  through  which 
and  like  starch  are  insoluble  in  the  cell  ^^^^^  ^nf hl'^'ma  -fieT"^ 
sap.    Before  being  consumed  by  the  plant 

as  food,  they  are  converted  by  means  of  enzymes  into  simpler 
and  soluble  forms,  such  as  glycerine  and  fatty  acids. 

The  vegetable  fats  are  not  only  important  animal  foods,  but 
are  used  in  many  ways  by  man  in  the  industries.  Rape  oil  and 
Corn  oil  are  much  used  for  lubricating  machinery.  Linseed  oil 
is  extensively  used  as  a  solvent  for  paint.  Palm  oil.  Cotton-seed 
oil,  and  Coconut  oil  are  made  into  substitutes  for  butter,  besides 
being  used  in  many  other  ways.  Castor  oil,  besides  being  used 
as  a  medicine,  lubricant,  and  illuminant,  is  used  in  manufacturing 
dyes.  The  use  of  Olive  oil  in  foods  is  well  known.  Many  of  the 
vegetable  oils  are  used  in  making  the  best  soaps. 


walls  of 


280 


LEA\^S 


am 


The  vegetable  proteins  are  of  many  kinds  and  they  vary  greatly 
in  physical  and  chemical  properties.     They  occur  as  crystals, 

granules,  or  in  solution 
in  the  vacuoles  of  the 
protoplasm,  or  in  inti- 
mate association  with 
the  protoplasm.  They 
are  present  to  some  ex- 
tent in  all  plant  cells, 
l)ut  are  more  prominent 
as  storage  products  in 
seeds,  where  they  are 
usually  associated  with 
starch  and  fats.  Some- 
times, as  in  the  aleurone 

Fig.  251. -Cross  section  through  grain  of  ^^^^"^  .^^  }^^  «^^^^1«' 
wheat  (Triticum  vulgar e);  p,  pericarp;  <,  testa;  there  IS  little  else  but 
a/,  aleurone  layer  containing  numerous  protein    proteins.  {I^W-     251.) 

grains;  n,  nucleus;  am,  starch  grains.     Enlarged    The  Leo^umes  store  COn- 
240  times.     After  Strasburger.  siderabTe     quantities     of 

proteins,  and  for  this  reason  some  of  them,  especially  the  Beans 
and  Peas,  are  very  desirable 
for  food.     {Fig.  252.) 

Proteins  differ  chiefly  from 
the  carbohydrates  and  fats 
in  that  they  contain  nitro- 
gen. They  are  known  as 
nitrogenous  foods.  In 
addition  to  nitrogen  thej- 
usually  contain  sulphur  and 
sometimes  phosphorus;  but 
nitrogen  is  the  chief  mineral 
constituent.  The  proteins 
are  extremely  complex,  as  the 
formnlf,      P     TT      M     O     d         TiG.  252.  —  Section  from  a  cotyledon 

lOrmUla       '--720Aln34iN.ii8U94Sk55        r      r)  u         •  e  11        •   •    a.  ii    i 

•     r  ofaPea,  showmgafewcells;  r.mtercellular 

lor  one  oi  tnem  inaicates.        space;    am,   starch   grains;    al,    aleurone 
The   steps  in    the    process    grains;  n,  nucleus.     Enlarged  240  times. 
by  which  the  photosynthetic   ^^^*«^  Strasburger. 
sugar  and  the  mineral  elements  are  formed  into  proteins  are  not  well 
known;  but  it  seems  clear  that  the  elements  of  the  sugar  are  first 


SECRETIONS  281 

combined  with  nitrogen  to  form  amino-compounds  or  amides, 
which  then  combine  with  themselves  and  other  mineral  elements 
to  form  proteins.  The  proteins,  like  the  starches  and  celluloses, 
are  thus  supposed  to  be  built  up  by  the  combining  of  simpler 
compounds.  That  amino-compounds  are  involved  in  the  for- 
mation of  proteins  is  suggested  by  the  fact  that  they  are  nearly 
always  present  in  plants,  and  also  by  the  fact  that  they  are  pro- 
duced when  proteins  are  decomposed.  Asparagin  C4H8O3N2  is 
one  of  the  most  common  amino-compounds  found  in  plants,  but 
a  number  of  others,  such  as  arginin,  tyrosin,  leucin,  and  trypto- 
phane, are  often  found  in  considerable  quantities  in  the  germinat- 
ing seeds  or  seedlings  of  plants. 

Plants  form  many  kinds  of  proteins,  but  most  of  them  belong 
to  one  of  the  general  classes  —  albumins,  globulins,  glutelins, 
gliadins,  or  nucleo- proteins.  The  albumins,  the  proteins  of 
which  the  white  of  an  egg  is  composed,  are  represented  in  Peas 
by  legumelin  and  in  Wheat  and  other  cereals  by  leucosin.  The 
globulins  are  common  in  the  Legumes,  legumin  being  the  chief 
one.  The  globulins  are  probably  the  most  abundant  of  the  re- 
serve proteins  in  all  seeds  except  cereals.  Glutenin  found  in  Wheat 
and  oryzenin  in  Rice  belong  to  the  glutelins.  The  gliadins  are 
found  in  the  cereals.  Gluten,  the  substance  upon  which  the  tenac- 
ity of  dough  depends,  consists  chiefly  of  glutelins  and  gliadins. 
The  nucleoproteins  occur  in  the  nucleus  of  the  cell  where  they 
are  an  important  constituent  of  chromatin.  Most  proteins  are 
insoluble  in  the  cell  sap,  and  need  to  be  digested  by  enzymes  into 
soluble  and  diffusible  forms,  such  as  proteoses,  peptones,  or  amino- 
compounds,  before  they  can  be  moved  through  the  plant. 

Secretions.  —  A  large  number  of  plant  substances,  differing 
widely  in  both  composition  and  function,  are  often  classed  as 
secretions.  The  important  classes  of  secretions  are  volatile  oils, 
glucosides,  alkaloids,  pigments,  and  enzymes.  Some  of  the 
secretions  accumulate  in  glands,  which  have  special  cells  for 
secreting  and  often  cavities  provided  for  holding  the  secretions, 
as  the  glands  on  the  leaves  of  the  Mints  and  in  the  skin  of  Oranges 
illustrate.  Some  accumulate  in  long  ducts  like  those  in  the  stems 
and  leaves  of  the  Milkweeds  and  Pines  and  are  secreted  by  the 
cells  around  these  ducts.  Many  of  the  secretions,  however,  are 
formed  in  cells,  in  which  secreting  is  only  a  minor  function,  and 
are  usually  found  in  solution  in  the  vacuoles  of  the  protoplasm. 


282  LEAVES 

The  volatile  oils  differ  not  onl}^  cheniicall}^  from  fatty  oils,  but 
also  in  being  volatile.  They  cause  most  of  the  odors  of  plants. 
Oil  of  Peppermint,  Sassafras,  Cinnamon,  Cloves,  Cedar,  and  the 
oil  of  the  Orange  rind  are  familiar  volatile  oils.  A  number  of 
uses  to  the  plant  have  been  assigned  to  the  various  volatile  oils, 
such  as  protection  against  destructive  organisms,  attraction  of 
insects  in  the  pollination  of  flowers,  and  serving  as  a  storage  form 
of  food.  Their  pleasant  odors  and  tastes  add  charm  to  the 
flowers  of  many  garden  plants,  and  before  the  chemists  learned 
to  make  many  of  them,  plants  were  our  chief  source  of  the  per- 
fumes and  essences  of  commerce.  Their  composition  suggests 
their  origin  from  the  photosynthetic  sugar,  since  nearly  all  of 
them  contain  only  carbon  and  hydrogen,  or  only  carbon,  hydrogen, 
and  oxygen. 

Closely  related  to  the  volatile  oils  are  a  number  of  substances, 
such  as  the  Pine  resins,  india  rubber,  gutta  percha,  camphor,  and 
asafetida,  which  are  important  commercially,  although  their  use 
to  the  plant  is  not  definitely  known.  From  the  Pine  resins,  which 
are  found  in  the  resin  ducts  of  Pines,  turpentine,  pine  tar,  rosin, 
and  pitch  are  obtained.  India  rubber  is  the  prepared  milk-juice 
obtained  from  a  number  of  trees  of  tropical  countries.  Gutta 
percha,  used  in  making  surgical  instruments,  in  fihing  teeth,  and 
in  a  number  of  other  ways,  is  obtained  also  from  the  milk-juice 
or  latex  of  a  number  of  plants.  Camphor  is  obtained  from  a 
number  of  tropical  trees  and  asafetida  from  a  group  of  herba- 
ceous plants. 

Glucosides  are  complex  substances  and  are  so  named  because 
many  of  them  contain  glucose  as  one  of  their  constituents.  They 
may  be  considered  storage  forms  of  food  since  they  yield  a  sugar 
when  broken  down.  Amygdalin,  the  bitter  substance  in  the 
seeds  of  the  Bitter  Almond,  is  one  of  the  best  known  glucosides. 
Its  formula  is  C20H27NO11,  and  when  decomposed  it  yields  glu- 
cose (C6H12O6),  hydrocyanic  acid  (HCN),  and  benzaldehyde 
(CeHsCHO).  Glucosides  vary  much  in  composition  and  con- 
sequently in  the  products  which  they  yield  when  decomposed. 
Thus  the  glucoside  coniferin  (C16H22O8),  found  in  coniferous  trees 
and  Asparagus,  yields  glucose  and  coniferyl  alcohol  (C10H12O3) 
when  decomposed. 

Glucosides  occur  in  all  parts  of  the  plant  and  are  especially 
abundant  in  the  parenchyma  cells  of  roots,  stems,  and  leaves. 


SECRETIONS  283 

On  account  of  the  hydrocyanic  acid  or  other  poisonous  substances 
contained,  many  of  the  glucosides  are  poisonous.  For  example, 
the  saponins,  which  are  present  in  Corn  Cockle  and  Cow  Cockle, 
are  poisonous  and  make  the  seeds  of  these  plants  very  objection- 
able impurities  of  the  small  grains.  The  mustards,  a  number 
of  which  have  poisonous  seeds,  contain  sinigrin,  a  poisonous 
glucoside.  In  the  seeds  of  some  Beans,  as  the  Burma  Bean,  there 
is  phaseolunatin,  a  poisonous  glucoside. 

Although  it  is  not  known  just  how  glucosides  are  formed  or 
their  exact  function  to  the  plant,  their  structure  shows  that  the 
photosynthetic  sugar  furnishes  most  of  the  elements.  Some 
of  them  may  be  directly  synthesized,  while  others  may  result 
from  the  decomposition  of  more  complex  compounds.  For  the 
decomposition  of  each  kind  of  glucoside  there  seems  to  be  a  special 
enzyme. 

The  alkaloids  constitute  another  group  of  substances  whose 
origin  and  function  are  obscure.  Some  of  the  familiar  ones  are 
caffein  and  thein  in  Coffee  and  Tea,  nicotine  in  Tobacco,  tnorphine 
from  the  Poppy,  quinine  from  the  bark  of  the  Cinchona  tree,  and 
stnjchnine  from  the  seeds  of  Nux  vomica.  In  containing  C,  H,  0,  N, 
they  show  a  close  relationship  to  the  amino-compounds.  The 
alkaloids  are  probably  protective  substances,  since  they  are 
often  unpleasant  to  the  taste  and  the  most  poisonous  group  of 
the  plant  substances.  In  the  preparation  of  drugs,  the  alkaloids 
have  a  very  important  place.  They  are  the  plant  poisons  which 
commonly  poison  stock  in  pastures  and  often  people  get  them 
by  mistake.  In  Poison  Hemlock  (Conium  maculatum)  and 
Water  Hemlock  {Cicuta  maculata)  there  is  the  poisonous  alkaloid, 
known  as  conin,  which  is  poisonous  to  stock  and  man.  In  the 
Nightshade  family,  of  which  Tomatoes  and  Irish  Potatoes  are 
representatives,  there  are  a  number  of  plants  which  contain 
atropine  and  solanin,  which  are  poisonous  alkaloids.  There  is 
a  large  number  of  plants,  many  of  which  are  common,  that  are 
poisonous  on  account  of  the  alkaloids  contained.  The  ptomaines, 
the  poisonous  substances  which  Bacteria  produce  in  the  decom- 
position of  meats,  are  alkaloids. 

Pigments  are  the  substances  upon  which  the  colors  in  plants 
depend.  Their  origin  is  obscure  and  in  some  cases  their  function 
is  not  known.  The  one  most  prominent  is  chlorophyll  with  the 
formula  often  given  as  C55H7206N4Mg.     Associated  with  chloro- 


284  LEA\'ES 

phyll  and  probably  decomposition  products  of  chlorophyll,  are 
carotin  (C40H56)  and  xanthophyll  (C40H56O2),  which  are  usually 
yellow  or  orange.  Carotin  is  so  named  because  of  its  abundance 
in  the  root  of  the  Carrot.  These  pigments  are  present  in  fruits, 
flowers,  and  autunm  leaves,  where  they  produce  the  yellow  and 
orange  colors.  Anthocyan,  a  pigment  whose  formula  is  not  well 
known,  occurs  dissolved  in  the  cell  sap  and  is  the  basis  of  the 
reds,  purples,  and  blues  in  plants,  being  red  when  the  cell  sap  is 
acid  and  blue  when  alkaline.  Besides  having  an  important  place 
in  determining  the  color  of  flowers  and  fruits,  it  often  occurs  in 
leaves  and  stems. 

In  addition  to  the  manufacture  of  food,  which  is  the  function 
of  chlorophyll,  the  pigments,  by  producing  the  showy  colors  of 
flowers,  assist  in  pollination.  They  also  add  to  the  attractiveness 
of  fruits  and  thereby  assist  in  the  dissemination  of  seeds.  Often 
the  yellow,  red,  and  blue  pigments  are  prominent  in  leaves  and 
other  structures  where  they  seem  to  have  no  function. 

The  enzymes  are  of  many  kinds  and  most  of  the  metabolic 
changes  in  cells  involve  the  action  of  enzymes.  They  are  the 
most  general  secretions  of  protoplasm  and  in  all  living  cells 
enzymes  of  some  kind  are  present.  So  far  as  chemical  analyses 
have  been  able  to  determine,  they  are  similar  to  proteins  in 
composition.  They  occur  dissolved  in  the  cell  sap  or  in  intimate 
relation  with  the  protoplasm,  but  often  diffuse  out  of  the  cell  and 
attack  surrounding  substances. 

Enzymes  are  specific  in  their  action  and  hence  there  are  almost 
as  many  kinds  of  enzymes  as  there  are  kinds  of  substances  to  be 
acted  upon.  There  is  a  class  of  enzymes  which  acts  on  proteins, 
one  that  acts  on  carbohydrates,  and  another  that  acts  on  fats. 
In  addition  there  are  enzymes  which  act  on  glucosides  and  other 
substances  of  minor  importance.  The  enzymes,  called  proteases, 
which  act  on  proteins  in  plants,  are  of  two  classes  —  ereptases  and 
peptases.  Peptases  have  been  found  in  a  number  of  plants  but 
they  are  not  so  generally  present  as  the  ereptases  are.  Ereptases 
apparently  break  up  proteins  more  completely  than  the  peptases 
do.  Bromelin  found  in  the  Pineapple  and  papain  in  the  Papaw 
(Carica  papaya)  are  two  well  known  peptases.  Papain  is  used 
in  making  digestive  tablets.  The  proteases  break  the  proteins 
into  soluble  forms,  such  as  proteoses,  peptones,  and  amino- 
compounds,  that  can  be  translocated  and  used  as  food. 


SEC^RETIONS 


285 


A  number  of  enzymes,  such  as  dinstase,  invertase,  maltase, 
zymase,  and  cytase,  are  involved  in  the  (Ugcstion  of  the  carbo- 
hydrates. Diastase,  which  is  especially  abundant  in  germinating 
seeds,  changes  starch  into  sugar.  Invertase  converts  cane  sugar 
into  glucose  and  fructose,  and  maltase  converts  maltose  into 
glucose.  Zymase,  well  known  as  a  secretion  of  the  Yeast  plant, 
converts  sugar  into  alcohol  and  carbon  dioxide.  Cytase  breaks 
cellulose  into  simpler  compounds. 

The  lipases  digest  the  fats  by  changing  them  into  fatty  acids 
and  glycerine,  in  which  form  the  fats  can  be  moved  in  the  plant 
and  consumed  as  food. 

For  the  glucosides  there  is   also   a   gioup   of  enzymes.     For 
amygdalin  there  is  amygdalase, 
and  for  other  glucosides  certain 
other   enzymes   which    decom- 
pose them. 

Although  we  know  that  en- 
zymes have  much  to  do  with 
the  metabolic  changes  in  plants, 
our  knowledge  of  enzymes  is 
comparatively  meager.  Inves- 
tigations on  the  kinds  of  en- 
zymes and  their  particular 
functions  are  much  needed  and  are  receiving  much  attention 
by  plant  chemists. 

Other  secretions  so  far  omitted  from  our  list  are  the  acids 
and  the  tannins.  Malic  acid,  oxalic  acid,  citric  acid,  tartaric  acid, 
and  a  number  of  others,  known  as  fruit  acids,  which  function  in 
determining  the  taste  of  fruits,  are  very  important  to  man  and 
assist  in  seed  dissemination  when  they  make  the  fruit  more 
pleasing  to  the  taste.  Some  of  the  organic  acids  often  form 
compounds  with  minerals  and  form  crystals.  Crystals  of  cal- 
cium oxalate  are  quite  common  in  plant  cells.  {Fig.  253.)  The 
tannins  are  bitter  astringent  substances  as  any  one  who  has 
tasted  a  green  Persimmon  well  knows.  They  occur  throughout 
plants,  but  are  more  abundant  in  the  bark.  Tannins  harden  the 
gelatine  in  skins,  and  before  the  chemists  provided  substitutes  bark 
was  extensively  used  in  tanning  leather.  Duo  to  their  astringent 
and  antiseptic  properties,  it  is  thought  that  they  protect  the  plant 
against  the  action  of  organisms,  such  as  Fungi  and  Bacteria. 


Fig. 


253.  —  Cells  containing  crystals 
of  calcium  oxalate. 


286  LEAVES 

Stored  in  all  forms  of  food  there  is  latent  energy,  which  is  trans- 
formed sunlight,  and  through  respiration  the  foods  are  broken 
into  simpler  compounds  with  the  release  of  energy,  which  is 
utilized  in  the  various  kinds  of  work  of  the  plant. 

Summary 

Leaves  may  be  classed  as  primary  and  secondary.  The 
primary  leaves,  represented  by  the  cotyledons,  are  mainly  storage 
organs  and  are  usually  short-lived.  The  secondary  leaves  form 
the  foliage  of  plants  and  are  the  food-making  organs.  The  form 
and  arrangement  of  leaves  vary  much,  but  usually  result  in  the 
best  exposure  to  light. 

The  chief  tissues  of  the  leaf  are  the  epidermis  consisting  of  pro- 
tective cells  and  stomata,  the  mesophyll  containing  the  working 
cells,  and  the  veins,  which  give  strength  and  supply  other  tissues 
with  water  and  salts  and  carry  away  the  manufactured  products. 

The  processes  taking  place  in  leaves  are  photosynthesis,  trans- 
piration, and  respiration.  Leaves  are  especially  adapted  to 
photosynthesis  because  of  their  green  tissue  and  exposure  to 
sunlight.  Upon  photosynthesis  the  carbohydrate  supply  of  the 
world  depends.  Respiration  is  not  peculiar  to  leaves;  for  it 
takes  place  in  all  living  cells,  but  can  be  easily  observed  in  leaves. 
Leaves  are  much  exposed  to  transpiration,  which  may  benefit 
the  plant  or  result  in  injury.  The  amount  of  water  lost  through 
transpiration  depends  upon  the  character  of  the  transpiring 
surface,  temperature,  humidity  of  the  air,  light,  and  velocity  of 
wind.  A  plant  may  be  protected  against  the  dangers  of  trans- 
piration by  having  its  transpiring  surface  modified  or  by  being 
able  to  supply  water  from  the  soil  or  storage  organs  in  sufficient 
quantities  to  meet  the  loss  through  transpiration. 

Leaves  may  have  become  modified  into  special  forms,  such  as 
scales,  tendrils,  thorns,  pitchers,  or  traps. 

Through  metabolism  the  photosynthetic  sugar  is  chemically 
combined  with  mineral  elements  to  form  proteins  and  proto- 
plasm, transformed  into  materials  which  constitute  a  frame- 
work, changed  into  storage  foods  and  into  various  other  kinds 
of  plant  products.  Through  respiration,  a  phase  of  metabolism, 
plants  obtain  chemical  energy  bv  releasing  the  latent  energ}^  of 
sugar  or  of  the  compounds  of  which  it  is  a  part. 


PART  II 

PLANTS  AS  TO   KINDS,   RELATIONSHIPS,  EVO- 
LUTION, AND   HEREDITY 


CHAPTER  XII 
INTRODUCTION 

Aside  from  some  references  to  Gymnosperms,  and  to  Yeast, 
Bacteria,  and  a  few  other  simple  plants,  Part  I  is  devoted  almost 
entirely  to  a  study  of  the  Morphology  and  Physiology  of  the  Flow- 
ering Plants.  The  Flowering  Plants  deserve  more  attention  than 
other  groups,  because  they  are  the  most  highly  developed,  most 
attractive,  and  are  the  chief  source  of  food,  fibers,  and  many  other 
products  related  to  the  welfare  of  mankind.  But  in  addition  to 
the  Flowering  Plants,  the  Plant  Kingdom  also  includes  many 
kinds  of  plants  which  do  not  have  flowers.  In  fact,  not  much 
more  than  half  of  the  233,000  or  more  species  of  known  plants  are 
Flowering  Plants.  About  us  are  many  kinds  of  plants  which  do 
not  have  flowers  and  some  of  them  are  also  of  much  economic 
importance.  The  Gymnosperms,  the  group  to  which  Pines, 
Spruces,  Firs,  and  some  other  trees  valuable  for  timber  belong,  do 
not  have  true  flowers  but  have  seeds,  and  are  almost  as  highly 
developed  as  the  Flowering  Plants.  The  Flowering  Plants  and 
Gymnosperms  constitute  the  group  called  Seed  Plants.  But 
there  are  many  kinds  of  plants,  which  are  often  referred  to  as  the 
simpler  plants,  that  do  not  even  have  seeds  and  some  of  these  are 
of  much  economic  importance.  Well  known  among  the  simpler 
plants  are  the  Ferns,  Mosses,  Algae,  Fungi,  and  Bacteria.  Both 
the  Fungi  and  the  Bacteria  are  important  economic  groups. 
The  Fungi  cause  most  of  the  plant  diseases  and  consequently 
much  destruction  and  loss  among  cultivated  plants.  Manj^  of 
the  Bacteria  are  indispensable  to  Agriculture,  for  they  decompose 
organic  compounds,  increase  the  nitrogen  of  the  soil,  and  do  other 
things  that  are  related  to  the  soil  fertility.  On  the  other  hand, 
the  Bacteria  cause  most  of  the  animal  diseases  and  these  forms 
we  have  to  combat. 

Some  of  the  simpler  forms,  like  the  Bacteria  and  many  Algae, 
are  unicellular  plants  and  hence  are  extremely  simple,  while  some, 
as  the  Ferns  illustrate,  have  complex  plant  bodies  and  are  com- 

289 


290  INTRODUCTION 

paratively  well  developed.  In  addition  to  the  many  kinds  of 
plants  now  living,  many  other  kinds  once  existed  but  are  now 
known  only  by  their  fossils. 

Among  the  kinds  of  plants,  including  both  the  living  and  fossil 
forms,  there  are  almost  all  degrees  of  complexity,  ranging  from 
the  simplest  unicellular  plants  to  the  most  highly  developed 
Flowering  Plants.  Although  varying  widely  in  complexity,  the 
various  kinds  of  plants  are  evidently  related  as  a  study  of  their 
structures  and  habits  reveals.  Scientists  believe  that  the  living 
forms  have  come  from  previously  existing  forms  and  hence  are 
related  through  a  common  ancestry.  They  have  originated 
through  the  process  known  as  evolution,  which  assumes  that  the 
first  plants  on  earth  were  extremely  simple  and  from  these  simple 
forms  the  more  complex  forms  arose.  In  response  to  a  changing 
environment  or  due  to  changes  arising  wholly  within,  the  simple 
forms  gave  rise  to  more  complex  forms,  which  in  turn  gave  rise  to 
forms  still  more  complex.  Thus  through  slow  changes  involving 
millions  of  years  the  highly  developed  forms  were  evolved. 

Evolution  has  generally  been  progressive,  giving  rise  to  forms 
with  higher  organization,  greater  perfection  of  parts,  and  in- 
creased efficiency  of  function.  Sometimes,  however,  evolution 
has  been  retrogressive,  and  forms  have  been  reduced  to  simpler 
forms,  through  becoming  more  simply  organized  and  less  efficient 
in  function.  For  example,  in  this  way  the  Fungi  are  supposed  to 
have  arisen  from  the  Algae.  Progressive  evolution  has  not  been 
direct  from  the  simplest  to  the  highest  organisms,  but  has  been 
along  many  lines  which,  although  usually  progressive,  have  been 
more  or  less  divergent  and  this  accounts  for  many  kinds  of 
organisms  among  both  plants  and  animals.  Animals  and  plants 
can  be  distinguished  in  their  higher  forms  on  the  basis  of  loco- 
motion, methods  of  getting  food,  character  of  the  skeleton,  and 
so  on,  but  in  their  simpler  forms  animals  and  plants  are  not  easily 
distinguished,  and  this  fact  suggests  that  plants  and  animals 
arose  as  diverging  lines  from  the  same  preexisting  organism.  A 
diagram  of  evolution  in  plants  looks  like  a  tree  with  many 
branches.  The  trunk  represents  the  main  line  and  the  branches 
the  diverging  lines  of  evolution.  The  lowest  branches  with  their 
sub-branches  represent  the  groups  of  the  simplest  plants  and 
their  relationships.  The  groups  of  Seed  Plants  and  their  rela- 
tionships are   represented   by  the  topmost  branches,  and  the 


CLASSIFICATION  OF  PLANTS  291 

branches  representing  other  groups  are  so  located  as  to  show  the 
rehitive  complexity  and  relationships  of  the  various  other  groups 
included  in  the  Plant  Kingdom. 

The  origin  of  a  plant  from  simpler  previously  existing  forms  is 
known  as  phylogeny,  while  the  series  of  changes  which  a  plant 
or  any  living  being  passes  through  in  attaining  a  mature  con- 
dition is  called  ontogeny.  Plants  are  classified  in  a  number  of 
ways,  but  chiefly  upon  their  phylogenetic  relationships.  An- 
other basis  of  considerable  importance  upon  which  plants  are 
classified  pertains  to  their  place  of  living  and  adjustments  to 
environment.     Relationships  of  this  kind  are  ecological. 

Part  II  is  devoted,  although  briefly:  first,  to  a  study  of  the 
structure,  habits,  economic  importance,  and  phylogenetic  rela- 
tionships of  the  plants  below  the  Flowering  Plants;  second,  to 
a  study  of  some  of  the  important  groups  of  Flowering  Plants  as 
to  their  phylogenetic  relationships  and  economic  importance; 
third,  to  a  consideration  of  plants  as  to  their  ecological  relation- 
ship; and  fourth,  to  a  special  study  of  evolution,  heredity,  and 
the  breeding  of  plants. 

Classification  of  Plants.  —  On  the  basis  of  descent  or  phylo- 
genetic relationships  the  Plant  Kingdom  is  divided  into  four 
groups  called  divisions.  Divisions  are  divided  into  classes, 
classes  into  orders,  orders  into  families,  families  into  genera,  and 
genera  into  species.  Species  are  the  units  in  the  phylogenetic 
classification.  Species  are  aggregates  of  individuals  which  are 
alike  in  their  important  characteristics  and  are  therefore  regarded 
as  the  same  in  kind.  Of  course  the  individuals  of  a  species  may 
vary  in  a  number  of  minor  features.  Thus  White  Oaks  vary 
much  in  size,  shape,  and  a  number  of  other  ways,  but  in  essential 
features  all  White  Oaks  are  alike  and  all  belong  to  the  species 
White  Oak.  Sometimes  some  of  the  individuals  of  a  species 
show  some  important  differences,  and  then  the  species  is  sub- 
divided, the  subdivisions  being  called  varieties,  strains,  or  races. 
Also  for  a  similar  reason  it  sometimes  happens  that  orders  are 
subdivided  into  suborders,  classes  into  subclasses,  and  families 
into  subfamilies. 

The  scientific  names  of  the  phylogenetic  groups  are  Greek  or 
Latin  terms  and  commonly  express  some  characteristic  of  the 
group.  With  some  exceptions  due  to  historical  causes,  the 
groups  are  named  according  to  a  rather  definite  plan.     The  names 


292  INTRODUCTION 

of  the  divisions  end  in  -phyta,  commonly  written  -phyte,  from 
the  Greek  word  phyton  which  means  plant.  The  names  of  classes 
commonly  end  in  -ineae  or  -eae  and  are  usually  derived  from  some 
important  group  included.  Thus  the  Lycopodineae  is  the  class 
containing  the  Lycopods,  and  the  Filicineae  is  the  class  contain- 
ing the  Ferns. 

The  names  of  orders  end  in  -ales,  and  orders  are  commonly 
named  from  some  prominent  family  included.  Thus  the  Rosales 
are  named  from  the  Rosaceae,  the  Rose  family.  Names  of  families 
usually  end  in  -aceae  and  are  commonly  derived  from  some 
prominent  genus,  as  for  example  the  Liliaceae,  which  is  the  Lily 
family.  The  names  of  genera  are  Latin  nouns  in  the  nominative 
case.  Thus  Quercus  is  the  Oak  genus,  Pyrus,  the  Apple  genus, 
and  Acer,  the  Maple  genus.  Species  have  two  names,  the  name 
of  the  genus  and  the  name  that  distinguishes  the  species.  For 
example,  Quercus  alba  is  the  White  Oak,  while  Quercus  rubra  is 
the  Red  Oak.  Quercus  is  the  name  of  the  genus,  and  the  terms 
alba  (the  Latin  term  for  white)  and  rubra  (the  Latin  term  for  red) 
name  the  species.  In  English  we  simply  change  the  terms  about 
and  say  White  Oak  instead  of  Oak  White. 

The  Divisions  of  the  Plant  Kingdom.  —  The  phylogenetic 
divisions  of  the  Plant  Kingdom  arranged  in  phylogenetic 
order  are  Thallophytes,  Bryophytes,  Pteridophytes,  and  Spermato- 
phytes. 

The  Thallophytes  are  the  simplest  plants  and  are  regarded  as 
the  lowest  and  most  primitive  from  the  standpoint  of  evolution. 
The  word  means  thallus  plants.  As  previously  stated  the 
ending  -phyte  always  means  plant.  Thallus  refers  to  the  fact 
that  the  plant  body  has  a  simple  organization.  It  is  not  differen- 
tiated into  roots,  stem,  and  leaves.  Bacteria,  Toadstools,  and 
Algae  are  familiar  Thallophytes.  The  plant  body  of  some  of 
them  consists  of  a  single  cell,  which  is  the  simplest  plant  body 
possible. 

The  Bryophytes  are  so  named  because  they  are  chiefly  Moss 
plants.  Besides  the  Mosses,  they  also  include  the  Liverworts. 
The  Bryophytes  have  better  organized  plant  bodies  than  the 
Thallophytes  and  are,  therefore,  considered  higher  in  the  scale 
of  evolution. 

The  Pteridophytes  are  so  named  because  they  include  the  Fern 
plants.     Most  Pteridophytes  are  Ferns,  but  this  group  includes 


SIMPLE   THALLOPHYTES   WITH   SPERMATOPllYTES      293 

some  plants  that  are  not  true  Ferns.  The  Pteridophytes  made 
much  advancement  in  developing  tissues  and  organs.  They 
have  roots,  stems,  and  leaves,  and  for  this  reason  are  regarded  as 
more  highly  developed  than  the  Bryophytes. 

The  Spermatophytes  are  the  Seed  Plants.  With  this  group  we 
are  most  familiar,  since  to  this  group  belong  the  trees,  shrubs, 
and  most  of  the  familiar  herbaceous  plants.  It  is  the  seed,  which 
is  one  of  their  contributions  to  evolution,  that  makes  many  of 
them  so  useful.  In  this  group  occurs  the  greatest  display'  of 
tissues  and  organs. 

The  Spermatophytes  consist  of  two  subdivisions,  Gymnosperms 
and  Angiosperms: 

The  Gymnosperms  (Gymnospermae),  as  the  term  signifies,  do 
not  have  their  seeds  enclosed.  These  are  the  evergreens,  such 
as  Pines,  Cedars,  Spruces,  Hemlocks,  Firs,  etc. 

The  Angiosperms  (Angiospermae),  as  the  term  signifies,  have 
their  seeds  enclosed.  This  refers  to  the  enclosing  of  the  seed  in 
an  ovary.  Nearly  all  of  the  cultivated  plants  belong  in  this 
group.     They  contribute  the  fruits. 

A  Comparison  of  Simple  Thallophytes  with  Spermatophytes.  — 
One  striking  difference  between  the  simplest  Thallophytes  and 
the  Spermatophytes  is  in  the  number  of  cells  of  which  the  plant 
is  composed.  The  simplest  Thallophytes  are  unicellular,  while 
the  Spermatophytes  are  extremely  multicellular.  A  second 
striking  difference  between  the  plants  of  the  two  divisions  is  in 
the  differentiation  and  specialization  of  cells  which  are  thereby 
fitted  to  perform  special  functions.  In  unicellular  Thallophytes 
one  cell  performs  all  of  the  different  kinds  of  work  that  the  plant 
has  to  do,  while  in  Spermatophytes  there  is  a  division  of  labor 
among  the  cells;  that  is,  Spermatophytes  have  tissues,  which 
are  groups  of  cells  especially  adapted  to  do  particular  kinds  of 
work. 

As  the  cells  of  multicellular  plants  become  differentiated  into 
tissues  and  thus  specialized  in  function,  they  lose  the  ability  to 
exist  independently.  Many  unicellular  plants  can  live  inde- 
pendently of  other  cells,  but  in  Spermatophytes,  the  life  of  a 
cell  in  most  cases  depends  upon  the  proper  adjustment  of  the  cell 
to  the  vital  processes  of  other  cells  of  the  plant  body.  Thus  the 
ability  of  a  cell  to  perform  many  functions  is  lost  in  becoming 
adapted  to  perform  one  function  well. 


294  INTRODUCTION 

In  organization  the  cells  of  Spermatophytes  do  not  differ  essen- 
tially from  those  of  most  Thallophytes.  Excepting  in  the  very 
lowest  forms,  the  cellular  structures  of  Thallophytes  are  similar 
to  those  of  the  Spermatophytes  as  a  study  of  the  mii  cellular 
Thallophyte  in  Figure  25^  will  show.  Tliis  one-celled  plant  is 
composed  of  protoplasm,  which  is  the  living  substance,  and  a 
wall,  which  encloses  the  protoplasm.  The  protoplasm,  as  in  the 
cells  of  higher  plants,  consists  of  nucleus  and  cytoplasm.  The 
nucleus,  usually  globular  in  shape,  is  en- 
closed by  a  nuclear  membrane  and  contains 
one  or  more  nucleoli  (small  globular  bodies) 
and  chromatin  (the  chunky  or  granular  sub- 
stance scattered  about  in  the  nucleus).  In 
Fig.  2  5  4. — A  addition  to  nucleoli  and  chromatin,  the  nu- 
one-celled  Thallo-  cleus  contains  nuclear  sap  (water  containing 
phyte,  Pleurococcus  sugar,  Salts,  and  other  substances  in  solu- 
vulgaHs.  n,  nucleus  ^^^^y  ^j^^  cytoplasm,  the  protoplasm  out- 
snowing     nuclear        .  ,        ^     ,  ,  .  ,  ,  , 

membrane  chro-  ^^^^^  ^^  *"®  nucleus,  is  vacuolate  and  has  its 
matin,  and  a  nucle-  outer  border  SO  modified  as  to  form  a  mem- 
olus.  c,  cytoplasm,  brane,  which,  unless  the  protoplasm  is 
m  which  there  is  a  shrunken,  is  tightly  pressed  against  the  cell 
arge  o  e  c  oro-  ^q\\  Water  and  solutions  enter  the  proto- 
plast.      X  800.        ,  ,  ,      ,  -         „  ,  ,         , 

From  Strasburger.  plasm  through  this  cell  membrane  by  the 
processes  of  osmosis  and  diffusion.  All  of 
these  cellular  structures  have  practically  the  same  function  here  as 
in  the  cells  of  the  higher  plants.  In  this  particular  unicellular  plant 
there  is  a  chloroplast,  which,  like  the  chloroplasts  in  the  food- 
making  cells  of  leaves,  is  a  special  protoplasmic  body  saturated 
with  a  green  pigment  (chlorophyll),  which  enables  it  by  utilizing 
the  sunlight  to  carry  on  photosynthesis,  that  is,  to  form  sugar 
from  carbon  dioxide  and  water. 

Although  consisting  of  a  single  cell,  this  plant  performs  most  of 
the  functions  which  the  most  highly  organized  plants  perform, 
but  in  a  simpler  way.  In  absorbing  water  and  mineral  elements 
directly  from  its  surroundings,  it  performs  the  function  of  roots. 
In  carrying  on  photosynthesis,  it  performs  the  function  of  leaves. 
By  dividing  it  gives  rise  to  new  individuals  and  thereby  performs 
the  function  of  reproduction,  which  is  the  function  of  flowers.  In 
such  a  simple  plant  there  is  no  function  comparable  to  that  of 
a  stem,  for  there  are  no  distant  parts,  such  as  leaves  and  roots, 


SIMPLE   THALLOPHVTES   WITfl   riPERMATOPHYTES       295 

to  be  connected  and  no  definite  position  which  the  pliuit  must 
maintain. 

It  is  now  clear  that  in  passing  from  the  unicellular  condition 
to  the  Spermatophyte  stage,  evolution  was  along  the  following 
lines:  First,  plants  became  multicellular;  second,  the  cells  con- 
stituting a  multicellular  plant  became  somewhat  differentiated  as 
to  function  and  structure;  third,  as  plants  became  more  multi- 
cellular, there  was  further  differentiation  which  eventually  re- 
sulted in  the  establishment  of  definite  structures  or  organs  fitted 
to  efficiently  perform  special  functions.  Such  structures  in  their 
most  highly  organized  form  are  the  leaves  organized  for  the 
manufacture  of  plant  food,  the  roots  organized  for  absorbing 
and  for  anchoring  the  plant,  the  flowers  organized  for  reproduc- 
tion, and  the  stem  organized  to  support  leaves,  flowers,  and  fruit 
in  the  air  and  sunshine. 

Of  course  the  organs  as  they  occur  in  Spermatophytes  did 
not  arise  suddenly,  but  they,  too,  underwent  a  gradual  process  of 
evolution,  at  first  arising  as  simple  structures  and  gradually 
becoming  more  complex  and  better  defined.  Through  the 
Thallophytes,  Bryophytes,  Pteridophytes,  and  Spermatophytes, 
including  both  living  and  extinct  forms,  the  organs  characteristic 
of  the  highest  type  of  Spermatophytes  gradually  arose. 


CHAPTER   XIII 

THALLOPHYTES 

Algae  (Thallophytes  with  a  Food-making  Pigment) 

General  Characteristics.  —  The  Algae  are  a  familiar  group  of 
Thallophytes,  for  in  nearly  every  lake,  pond,  and  stream,  and 
along  the  sea  coast  some  forms  of  them  can  be  found.  They 
commonly  appear  in  fresh  water  as  a  green  scum  or  as  floating 
mats  of  green  threads  on  or  near  the  surface  of  the  water.  They 
often  occur  in  abundance  in  watering  troughs,  and  sometimes 
become  troublesome  by  clogging  sewers  and  water  mains.  Along 
the  sea  coast  occur  the  large  brown  and  red  forms  known  as 
Seaweeds. 

Algae  are  of  some  economic  importance.  The  Seaweeds  are 
much  used  as  food  in  some  countries,  especially  in  Japan,  and 
from  some  Seaweeds  iodine  and  potassium  are  extracted.  Along 
the  Pacific  Coast  of  the  United  States,  Seaweeds  are  an  impor- 
tant source  of  potassium  for  fertilizers.  However,  the  interest 
in  the  study  of  Algae  is  not  due  so  much  to  their  economic 
importance  as  it  is  to  the  fact  that  a  knowledge  of  them  is 
essential  to  an  understanding  of  the  evolution  of  the  higher 
plant  forms. 

Although  Algae  are  water  plants,  not  all  Algae  live  in  the  water, 
for  there  are  some  forms  which  live  on  moist  soil  or  rocks  where 
water  is  easily  obtained,  and  a  few  exceptional  forms,  such  as 
those  that  live  on  the  bark  of  trees,  have  very  dry  surroundings 
much  of  the  time.  Algae  differ  from  other  groups  of  Thallophytes 
in  having  food-making  pigments  by  which  they  make  their  car- 
bohydrates. Consequently,  they  are  not  saprophytes  or  para- 
sites, that  is,  plants  which  have  to  depend  directly  upon  other 
plants  for  food,  but  are  equipped  to  live  independently.  Among 
them  there  is  a  wide  range  of  variation  in  plant  body  and 
methods  of  reproduction,  and  four  groups  of  Algae  are  commonly 
recognized  —  Blue-green,  Green,  Brown,  and  Red  Algae. 


BLUE-GREEN  ALGAE  297 

Blue-green  Algae.     Cyanophyceae 

The  Blue-green  Algae  are  the  simplest  forms  of  Algae  and  are 
the  simplest  known  plants  that  make  their  own  food.  They 
are  so  named  because  of  their  bluish  green  color  which  is  due 
to  the  presence  of  chlorophyll  and  a  blue  pigment  called  Phyco- 
cyanin.  Although  their  size  is  microscopical,  they  form  aggre- 
gations that  are  often  quite  conspicuous.  There  are  about  1200 
species  of  Blue-green  Algae,  and  they  are  widely  distributed, 
occurring  nearly  everywhere  in  fresh  and  salt  water  and  also  on 
wet  soil,  rocks,  and  logs.  On  wet  surfaces  they  form  bluish 
green  slimy  layers  or  jelly-like  lumps,  and  in  sluggish  streams 
and  ponds  they  form  bluish  green  scums  or  mats  which  float  on 
or  near  the  surface  of  the  water.  They  thrive  best  where  there 
is  organic  matter  and  consequently  prefer  stagnant  to  running 
waters.  Some  forms  are  so  resistant  to  heat  that  they  can  live 
in  hot  springs  where  the  temperature  is  near  the  boiling  point 
of  water.  Some,  called  endophytes,  live  in  the  cavities  of  some  of 
the  more  highly  organized  plants,  such  as  the  Liverworts  and 
Ferns.  Some  are  associated  with  Fungi  in  the  formation  of 
Lichens.  The  Blue-green  Algae  are  of  only  slight  economic  im- 
portance. When  allowed  to  accumulate,  they  impart  offensive 
odors  to  water  supplies,  but  are  easily  controlled  by  use  of 
copper  salts.  It  is  claimed  that  livestock  are  sometimes  killed 
by  drinking  water  that  has  become  foul  with  Blue-green  Algae. 

The  plant  body  in  the  Blue-green  Algae  is  a  single  cell  or  a 
colony  of  cells  so  joined  as  to  form  a  filament  or  plate.  When 
cell  division  is  in  only  one  direction  and  the  cells  formed  do  not 
separate,  then  as  a  result  of  a  number  of  successive  cell  divisions 
a  chain  or  filament  of  cells  is  formed.  When  cell  division  is  in 
more  than  one  direction  and  the  cells  do  not  separate,  then 
colonies  of  other  shapes  are  formed.  Colonies,  although  they 
may  resemble  nmlticellular  plants,  are  aggregates  of  essentially 
independent  cells.  One  notable  feature  of  the  plant  body  of  the 
Blue-green  Algae  is  the  secretion  of  a  gelatinous  substance  which 
forms  a  sheath  about  the  plant.  As  plants  grow  and  multiply, 
the  gelatinous  secretion  accumulates  and  commonly  forms  a 
matrix  which  holds  the  plants  together  in  slimy  layers  or  jolly- 
like lumps.  The  gelatinous  sheath  holds  water  and  thus  protects 
the  plants  from  drying  out.     Another  notable  feature  of  this 


298 


THALLOPHYTES 


group  pertains  to  the  organization  of  the  cell.  In  a  few  of  the 
most  highly  developed  forms  the  protoplast  is  pretty  well  organ- 
ized, but  in  most  Blue-green  Algae  the  nucleus  and  cytoplasm 
are  not  clearly  differentiated  and  there  are  no  chloroplasts.  The 
chlorophyll  and  other  pigments  are  diffused  through  the  cyto- 
plasm and  sometimes  throughout  the  entire  protoplast. 

A  simple  form  of  Blue-green  Algae  is  Gleocapsa  shown  in  Figure 
255.  This  plant,  which  lives  mostly  on  wet  rocks,  consists  of  a 
single  globular  cell  with  a  rather  prominent  gelatinous  sheath 
and  is  about  as  simple  as  a  plant  can  possibly  be.     By  the  divi- 


FiG.  255.  —  Gleocapsa,  one  of  the 
simplest  of  the  Blue-green  Algae.  A, 
single  individual  enclosed  in  a  heavy 
gelatinous  sheath  and  beginning  to 
divide.  B  and  C  show  how  the 
plants  as  they  multiply  are  held  to- 
gether by  the  gelatinous  sheath. 
X  540.     After  Strasburger. 


Fig.  256.  —  Portions  of  three  fil- 
aments of  Oscillatoria.  At  the  left 
one  cell  in  the  filament  has  died, 
resulting  in  segmenting  the  fila- 
ment.    X  540. 


sion  of  the  cell  new  individuals  are  formed,  which  are  held 
together  in  loose  aggregations  by  the  gelatinous  secretion  from 
their  walls. 

One  of  the  common  colonial  forms  is  Oscillatoria,  of  which  there 
are  about  100  species  {Fig.  256).  They  form  bluish  green  felt- 
like mats  in  fresh  and  salt  water,  and  bluish  green  layers  on  moist 
soil.  The  colony  is  a  filament,  consisting  of  a  large  number  of 
short  cylindrical  cells  joined  end  to  end  and  enveloped  in  a  thin 
gelatinous  sheath.  Usually  the  filaments  occur  together  in  large 
numbers,  and  often  there  is  enough  of  the  gelatinous  secretion  to 
hold  them  together  in  loose  aggregations.  A  characteristic 
feature  of  the  plant,  as  the  name  suggests,  is  the  swaying  and 
revolving  movement  of  the  filament,  which  sometimes  resembles 


BLUE-GREEN   ALGAE 


299 


a  tiny  worm  in  its  creeping  and  bending  to  one  side  and  then  the 
other.  This  movement  indicates  that  the  cells  of  the  colony  of 
Oscillatoria  work  together  as  a  unit  and  thus  the  many-celled 
colony  takes  on  the  character  of  a  many-celled  plant  where  the 
cells  are  closely  associated  in  the  activities  of  the  plant. 

Another  filamentous  form  (Fig.  257)  is  Nostoc,  which  is  common 
in  fresh  water  and  on  moist  soil.  In  this  plant  the  cells  are 
rounded  and  the  filament  re- 
sembles a  chain  of  beads.  Nostoc 
secretes  an  extraordinary  amount 
of  gelatinous  substance  and  forms 
jelly-like  lumps  in  which  a  large 
number  of  the  plants  are  held. 
These  jelly-like  masses  are  often 
more  or  less  rounded,  and  are  of 
various  sizes  up  to  that  of  a 
marble  or  even  larger.  When 
growing  on  soil,  they  often  swell 
up  and  glisten  after  a  rain,  on 
which  account  they  have  been 
called  "  fallen  stars." 

In  Nostoc  there  is  some  differ- 
entiation of  cells.  At  intervals 
in  the  filament  ordinary  working 
cells  enlarge,  lose  their  contents, 
and  thicken  their  walls.  Being 
larger  in  size  and  almost  colorless, 
they  are  quite  distinct  from  the 
other  cells  of  the  filament,  and 
thus  divide  the  filament  into  sec- 
tions called  harmogonia.  These  special  cells,  called  heterocysts, 
seem  to  be  concerned  with  the  multiplication  of  filaments,  for 
it  has  been  observed  that  the  harmogonia  break  loose  at  the 
heterocysts,  wriggle  out  through  the  jelly-like  matrix,  and  de- 
velop new  filaments. 

Another  special  kind  of  cell  formed  in  Nostoc  is  the  resting  cell, 
which  is  formed  when  periods  unfavorable  for  the  growth  of  the 
plant  appear.  In  this  case  certain  cells  of  the  filament  enlarge, 
accumulate  food,  and  thicken  their  walls.  These  cells  are  able 
to  endure  cold,  drought,  and  other  conditions  which  are  destruc- 


FiG.  257.  —  Nostoc.  At  the 
left  are  jelly-like  lumps  of  Nostoc 
consisting  of  numerous  colonies. 
About  natural  size.  At  the  right 
is  a  single  colony,  showing  the 
gelatinous  sheath  and  the  hetero- 
cysts, the  large  cells  shown  empty, 
which  segment  the  filament  into 
harmogonia.     X  540. 


300 


THALLOPHYTES 


tive  to  the  ordinary  cells  of  the  filament;  and,  when  favorable 
conditions  for  growth  return,  the  protoplast  of  the  resting  cell 
breaks  through  the  heavy  wall  and  develops  a  new  filament. 

In  Rivularia  (Fig.  258),  another  filamentous  form,  the  filament 

is  apparently  differentiated  into  a  basal  and  apical  region.     A 

heterocyst  is  the  basal  cell  and  the  cells  decrease  in  size  toward 

the  apex,  so  that  the  filament  has  a  whip-like 

appearance. 

Besides  the  features  just  mentioned  in  con- 
nection with  the  plant  body,  there  are  some 
other  minor  ones  which  some  particular  species 
of  Blue-green  Algae  have.  For  example,  in 
one  species  the  cells  of  the  colony  arrange 
themselves  so  as  to  maintain  a  regular  rec- 
tangle. In  some  forms  the  colony  forms  a 
branched  filament. 

Food  is  manufactured,  and  water  and  min- 
eral matters  are  absorbed  by  these  simple 
plants  in  essentially  the  same  way  as  in  the 
more  complex  plants,  but  each  cell  must 
manufacture  food  and  absorb  water  and 
mineral  matters  for  itself.  Since  these  plants 
live  in  water  or  on  a  moist  substratum,  they 
are  able  to  absorb  water  and  mineral  matters 
from  their  immediate  surroundings.  Having 
chlorophyll,  they  are  able  to  carry  on  photo- 
synthesis and  thereby  provide  themselves 
with  carbohydrates.  Although  the  function 
of  phycocyanin  is  not  known,  it  is  probable 
that  it  assists  some  in  connection  with  photo- 
synthesis. Sometimes  there  is  an  additional  reddish  pigment 
developed,  which  may  have  something  to  do  with  enabling  the 
plant  to  utilize  the  sun's  rays  in  the  manufacture  of  food.  The 
reddish  pigment  is  so  abundant  in  a  few  forms  that  the  plants 
appear  red  in  mass,  as  in  one  group  which  forms  floating  colonies 
in  salt  water  and  has  given  the  name  to  the  Red  Sea. 

Reproduction  in  the  Blue-green  Algae  is  chiefly  by  cell  division. 
They  form  no  sex  cells  and,  therefore,  depend  entirely  upon 
vegetative  methods  of  reproduction.  By  cell  division  new  cells 
are  formed,  which  may,  according  to  the  species,  separate  as  new 


Fig.  258.  — a 
single  colony  of 
Rivularia  consist- 
ing of  a  large  hete- 
rocyst and  many 
vegetative  cells 
which  decrease  in 
size  away  from  the 
heterocyst.    X  540. 


GREEN  ALGAE  301 

plants,  as  in  Glcocapsa,  or  remain  as  a  part  of  a  close  colony,  as 
in  Oscillatoria  and  other  forms  where  the  cells  of  a  colony  are 
closely  associated.  In  filamentous  forms  the  method  of  multi- 
plying filaments  by  means  of  harmogonia  may  be  classed  as  a 
method  of  reproduction.  In  this  case  a  filament  breaks  into 
segments  which  separate  and  establish  new  filaments.  The 
filament  may  be  segmented  by  heterocysts  or  by  the  death  of 
ordinary  working  cells. 

The  simplicity  of  plant  body,  cellular  structures,  and  methods 
of  reproduction  makes  the  Cyanophyceae  the  simplest  of  all 
groups  of  independent  plants  now  in  existence.  The  absence 
of  chloroplasts  and  a  well-defined  nucleus  and  cytoplasm  clearly 
distinguishes  them  from  other  groups  of  independent  plants. 
But  in  the  group  some  advancement  is  shown.  The  formation 
of  a  colony  in  which  the  cells  are  closely  associated  looks  forward 
toward  the  formation  of  multicellular  plants  in  which  the  cells 
are  very  intimately  associated.  Also  the  differentiation  of  the 
cells  of  a  colony  into  ordinary  working  cells,  heterocysts,  and 
resting  cells  suggests  the  differentiation  of  cells  in  multicellular 
plants  into  tissues. 


Green  Algae  (Chlorophyceae) 

The  Green  Algae  are  the  Algae  most  commonly  seen  in  our  lakes, 
ponds,  and  streams.  They  usually  have  only  one  pigment,  chloro- 
phyll, and  their  green  or  yellow-green  color  is  usually  quite 
distinct  from  that  of  the  Blue-green  Algae.  Some  of  the  Green 
Algae  are  microscopic  and  some  form  colonies  or  multicellular 
plant  bodies  that  are  clearly  visible  to  the  naked  eye.  Although 
they  are  small  plants,  large  numbers  of  them  commonly  occur 
together,  forming  scums  or  tangles  of  filaments  that  are  conspicu- 
ous. Most  of  them  live  in  the  water  but  some  live  on  moist 
earth,  rocks,  or  wood,  and  a  few  forms  can  endure  periods  of 
drought.  A  few  forms  live  in  salt  water,  but  nearl}^  all  are  fresh 
water  plants. 

The  Green  Algae  differ  from  the  Blue-green  Algae  not  only 
in  color  but  also  in  a  number  of  other  ways.  Gelatinous  sub- 
stances are  secreted  in  abundance  onl}^  in  the  lowest  forms  of 
the  group,  and  consequently  Green  Algae  do  not  commonly 
form    gelatinous    masses.      They   have    chloroplasts,    and    the 


302  THALLOPIIYTES 

nucleus  is  well  organized  and  quite  distinct  from  the  cytoplasm. 
In  some,  the  cells  of  the  colony  have  their  protoplasts  joined 
by  protoplasmic  strands  and  have  thus  become  so  closely  asso- 
ciated that  they  constitute  a  multicellular  plant.  Some  repro- 
duce entirely  by  cell  division,  but  many  of  them  have  more 
specialized  methods  of  reproduction.  Many  Green  Algae  form 
swimming  cells  called  zoospores,  each  of  which  is  able  to  pro- 
duce a  new  plant  directly.  Others  also  form  gametes  or  sex 
cells  which  fuse  and  form  a  cell  that  develops  a  new  plant  either 
directly  or  indirectly.  The  simplest  gametes  occurring  in  the 
group  are  alike  as  to  size,  structure,  and  behavior  and  are  called 
isogametes.  When  isogametes  pair  and  fuse,  a  cell  called  a 
zygospore  or  zygote  is  formed,  and  this  spore  may  form  zoospores 
or  develop  a  new  plant  directly.  The  fusion  of  similar  gametes 
is  called  conjugation.  The  more  advanced  Green  Algae  form 
morphologically  unlike  gametes  called  heterogametes,  of  which  the 
large  ones  are  called  eggs  and  the  small  ones  are  called  sperms. 
The  spore  formed  by  the  fusion  of  an  egg  and  a  sperm,  that  is,  by 
the  fusion  of  unlike  gametes,  is  called  an  oospore  and  the  fusion  is 
called  fertilization.  Often  the  gametes  are  produced  in  special 
organs  called  sex  organs.  It  is  evident  that  the  Green  Algae 
resemble  the  higher  plants  much  more  than  do  the  Blue-green 
Algae  and  are,  therefore,  considered  more  advanced.  It  is  sup- 
posed that  from  plants  like  the  Green  Algae  the  higher  plants 
have  come. 

Among  the  Green  Algae  there  is  much  diversity  in  character 
of  plant  body  and  methods  of  reproduction.  About  9000 
species  are  known  and  these  are  commonly  grouped  into  five 
orders  —  Volvocales,  Protococcales,  Confervales,  Conjugales,  and 
Siphonales. 

Unicellular  Motile  Green  Algae  (Volvocales).  —  These  Green 
Algae  are  regarded  as  one-celled  plants,  although  some  of  them 
form  colonies  of  considerable  size  and  complexity.  They  live  in 
the  water  and  chiefly  in  fresh  water.  Their  vegetative  cells  have 
cilia  and  swim  about  like  the  lower  animals.  It  is  this  motile 
habit  that  distinguishes  them  from  other  Green  Algae.  On  ac- 
count of  their  motility  and  some  animal-like  structural  features, 
they  are  sometimes  regarded  as  animals.  They  are  microscopic 
plants,  but  some  of  them  form  colonies  that  are  sometimes  visible 
to  the  naked  eye. 


CHLAMYDOMONAS 


303 


Chlamydomonas.  —  In  Chlamydomonas  {Fig.  259),  which  is 
regarded  as  one  of  the  simplest  of  the  Volvocales,  the  habit  of 
colony  formation  is  lacking  and,  therefore,  each  individual  swims 
about  independently.  This  plant  is  common  in  fresh  water  and 
when  seen  swimming  about  under  the  microscope  might  be  mis- 
taken for  a  protozoan,  a  one-celled  animal  which  it  resembles. 

The  Plant  body  consists  of  a  more  or  less  globular  protoplast 
closely  invested  by  a  thin  membrane  through  which  the  two  long 
cilia  project  at  the  forward  end. 
There  is  a  large  cup-shaped  chloro- 
plast,  in  which  there  is  a  protein 
body  called  pyrenoid.  The  nucleus 
is  in  the  cup  of  the  chloroplast;  at 
the  base  of  the  cilia  are  two  con- 
tractile vacuoles;  and  not  far  from 
these  is  the  red  pigment  spot  or 
eye  spot  which  is  supposed  to  be 
sensitive  to  light  and,  therefore,  of 
some  use  in  directing  the  movements 
of  the  individual.  In  certain  species 
a  bright  red  pigment  is  often  so 
abundant  that,  when  the  plants  are 
numerous,  they  cause  pools  to  ap- 
pear red  and,  when  blown  over  the 
snow,  produce  the  "  red  snow  "  of 
arctic  and  alpine  regions. 

In  the  number  of  cells  constituting 
the  plant  body,  Chlamj^domonas  is 

as  simple  as  any  of  the  Blue-green  Algae,  but  in  having  a  chloro- 
plast and  well-defined  nucleus  and  cytoplasm,  it  shows  considerable 
advancement. 

Reproduction  takes  place  by  means  of  zoospores  and  gametes. 
In  forming  zoospores  the  plant  becomes  quiescent  and  the  proto- 
plast divides  into  two  or  more  ciliated  cells  which  are  miniatures 
of  the  parent.  These  daughter  cells  or  zoospores  escape  from 
the  mother  cell  and  enlarge  to  the  parent  size.  Under  certain 
conditions  the  protoplast  may  form  many  small  zoospore-like 
cells  which  escape  from  the  mother  plant  and  fuse  in  pairs  to 
form  resting  zygospores  which  later  form  new  plants.  Since 
these  small  zoospore-like  cells  fuse,  they  are  gametes  or  sex  cells 


Fig.  259. — Chlamydomonas, 
a  simple  motile  Green  Algae. 
At  the  left  an  individual,  show- 
ing the  cilia,  the  large  cup- 
shaped  chloroplast  (c)  con- 
taining a  pyrenoid,  the  nucleus 
(n),  the  two  pulsating  vacuoles 
(p),  and  the  red  pigment  spot 
represented  by  a  black  dot  near 
the  pulsating  vacuoles.  At 
the  right  an  individual  which 
has  formed  two  zoospores. 
X  300. 


304  THALLOPHYTES 

and,  since  they  are  alike,  they  are  isogametes.  The  zygospore, 
a  spore  formed  by  the  fusion  of  similar  gametes  as  the  prefix 
{zygo)  suggests,  is  commonly  a  well-protected  spore  and,  there- 
fore, able  to  resist  conditions  that  are  destructive  to  the  zo- 
ospores or  vegetative  cells  of  the  plant.  The  approach  of  unfavor- 
al)le  conditions  commonly  induces  the  formation  of  gametes  and 
zygospores.  The  zygospore  remains  dormant  until  favorable 
conditions  return  and  then  produces  a  new  plant.  The  zygo- 
spore is,  therefore,  a  stage  in  the  round  of  life  in  which  the  plant 
is  able  to  survive  unfavorable  conditions. 

Gametes  are  supposed  to  be  zoospores  that  are  too  small  to 
function  alone.  By  pairing  and  fusing,  the  energies  of  two 
gametes  are  combined  in  a  zygospore  which  is  able  to  produce 
a  new  plant  that  neither  of  the  gametes  could  produce  alone. 
Thus  the  zygospore  may  also  be  regarded  as  a  cell  in  which  gam- 
etes combine  their  energies,  so  that  they  may  be  effective  in  pro- 
ducing new  plants.  There  are  two  things  which  indicate  that 
gametes  are  miniature  zoospores.  -First,  gametes  and  zoospores 
grade  into  each  other  in  size.  Second,  it  has  been  observed  that 
small  zoospores  may  fuse  and,  therefore,  behave  as  gametes  when 
poorl}^  nourished,  or  grow  directly  into  new  plants  and,  therefore, 
function  as  zoospores  when  well  nourished.  Thus  a  zoospore-like 
cell  may  be  a  zoospore  or  gamete  according  to  conditions.  Such 
is  the  evidence  supporting  the  theory  that  sexuality  arose  through 
the  fusion  of  zoospores  which,  on  account  of  size,  or  conditions 
of  light,  temperature,  food,  etc.,  were  unable  to  function  alone. 
From  this  simple  isogamous  sexuality  the  more  complex  heterog- 
amous  forms  of  sexuality  have  followed.  Even  in  some  forms 
of  Chlamydomonas,  the  gametes  pairing  often  differ  some  in  size 
and,  therefore,  suggest  heterogamous  sexuality. 

Pandorina.  —  The  colony,  which  is  one  of  the  notable  features 
of  the  Volvocales,  varies  widely  in  different  genera,  ranging  from 
16  cells  or  less  up  to  20,000  or  more.  Pandorina,  shown  in  Figure 
260,  is  one  of  the  forms  producing  simple  colonies. 

The  cells  or  individuals  of  which  the  colony  of  Pandorina  is 
formed  are  similar  in  structure  to  Chlamydomonas.  Commonly 
the  spherical  colony  consists  of  16  individuals,  held  together  in 
a  mucilaginous  matrix. 

Reproduction  differs  in  some  ways  from  that  of  Chlamydomonas 
on  account  of  the  colony  formation.      Any  individual  of   the 


VOLVOX 


305 


colony  may  divide  into  IG  zoosporc-likc  cells  which  remain 
together,  escape  from  the  mother  colony,  and  thus  become  a  new 
colony.  The  gametes  are  formed  in  essentially  the  same  way  as 
the  individuals  of  the  new  colonies,  but  they  separate  and  thus 
swim  about  independently  after  leaving  the  mother  colony. 
When  the  zygospore  germinates,  as  shown  in  Figure  260,  there 
results  a  new   colony  which   has    only  to  grow  to  adult  size. 


d      '^  e 

Fig.  2G0.  —  Pandorina  77wrum.  a,  Motile  colony  ordinarily  consisting  of 
sixteen  motile  cells  (X  475);  b,  colony  in  which  the  cells  have  fornaed 
daughter  colonies  (X  475);  c,  two  gametes  fusing;  d,  zygospore;  e,  zygo- 
spore germinating  and  forming  a  new  colony.  Redrawn  with  modifications 
from  Oersted. 

Among  the  gametes  there  is  often  considerable  variation  in  size  and 
motility,  some  being  smaller  and  more  active  than  others.  The 
gametes  pair  and  fuse  regardless  of  their  size,  and,  when  gametes 
that  are  unlike  happen  to  pair,  there  is  a  suggestion  of  heterog- 
amy, although  there  is  no  distinct  differentiation  of  gametes  as 
occurs  in  plants  where  heterogamy  is  well  established. 

Volvox.  —  The  highest  expression  of  colony  formation  is  reached 
in  forms  like  Volvox  (Fig.  261),  where  the  colony  contains  thou- 
sands of  individuals  held  together  in  a  gelatinous  matrix  and 
so  arranged  as  to  form  a  hollow  sphere.  The  colonies  of  Volvox 
are  often  as  large  as  a  pin  head  and  hence  visible  to  the  naked  eye. 
The  two  cilia  of  each  individual  project  from  the  colony,  and  by 
the  lashing  of  the  cilia  the  colony  moves  through  the  water  bj'  a 
revolving  motion.  One  can  often  see  them  slowly  moving  about 
in  ditches,  ponds,  and  sometimes  in  tanks  in  greenhouses.  A 
microscopical  study  of  the  colony  shows  that  the  individuals  of 
the  colony  are  connected  by  protoplasmic  strands,  and  hence  so 


306 


THALLOPHYTES 


vitally  related  that  the  colony  of  Volvox  may  be  regarded  as  a 
multicellular  individual  rather  than  a  colony. 

Reproduction  presents  some  interesting  features.  At  first  all 
cells  of  the  colony  are  alike,  but  later  considerable  differentiation 
among  cells  occurs.     Some  cells  of  the  colony  enlarge  and  pass 


Fig.  261.  —  Volvox.  In  the  colony  {Volvox  aureus)  the  smaller  cells  bear- 
ing two  cilia  are  the  vegetative  cells,  the  enlarged  cells  (a)  contain  sperms, 
and  the  enlarged  cells  (o),  varying  in  size  and  stages  of  development,  are 
eggs  (X  300).  Below  and  at  the  right  is  a  sperm,  and  below  and  at  the 
left  is  a  oospore  of  Volvox  globator.     After  West. 


into  the  hollow  of  the  sphere  where  they  form  new  colonies  which 
escape  and  grow  to  adult  size.  Sexual  reproduction  in  Volvox 
is  heterogamous,  for  two  distinct  kinds  of  gametes  are  involved. 
Some  of  the  cells  enlarge,  lose  their  cilia,  and  become  filled  with 
food.  They  are  the  female  gametes  or  eggs.  Other  cells  of  the 
colony  form  numerous  small  motile  gametes  or  sperms  which  seek 
the  eggs  and  fuse  with  them.  Fertilization,  as  this  fusing  is 
called  since  the  gametes  are  differentiated  into  eggs  and  sperms, 


PLEUROCOCCUS 


307 


occurs  within  the  hollow  of  the  sphere.  The  spore  formed  from 
the  fusion,  now  known  as  an  oospore  (meaning  eggspore),  forms  a 
new  colony  upon  germination. 

There  is  much  advantage  gained  by  differentiating  gametes. 
The  egg,  owing  to  its  size  and  loss  of  motihty,  can  store  much 
food  for  the  next  generation.  The  smallness  of  sperms  makes  it 
possible  for  large  numbers  of  them  to  be  produced,  and  promotes 
their  movements  through  water. 

In  summarizing  the  Volvocales,  the  following  features  are  the 
notable  ones.  The  plant  body  consists  of  a  single  motile  cell 
having  a  chloroplast  and  well-defined  nucleus  and  cytoplasm. 
Some  swim  about  independently,  but  the  formation  of  colonies 
is  a  marked  feature  of  the  group,  and  the  colonies  range  from 
simple  to  complex  ones.  By  the  division  of  cells  new  individuals 
and  new  colonies  are  formed.  Sexual  reproduction  advances 
from  isogamy  to  heterogamy.  As 
in  the  Blue-green  Algae,  the  forma- 
tion of  colonies  is  a  step  toward 
the  formation  of  multicellular  in- 
dividuals. 

Unicellular  Non-motile  Green 
Algae  (Protococcales).  —  In  con- 
trast to  the  Volvocales,  the  absence 
of  cilia,  except  on  reproductive  cells, 
is  a  notable  feature  of  this  group. 
Some  plants  of  this  order  are  very 
common  on  damp  soil,  walls,  and  on 
the  bark  of  trees,  where  they  are 
often  exposed  to  long  periods  of 
drought.  Most  of  the  group  are 
aquatic  and  occur  mainly  in  fresh 
water.  Some  enter  into  the  forma- 
tion of  Lichens.  Others  are  endo- 
phytic, living  in  the  intercellular 
spaces  of  other  plants,  and  some  give  the  green  color  to  certain 
animals,  such  as  the  hydra  and  fresh-water  sponge,  which  eat 
them.  They  show  considerable  variation  in  their  habit  of  form- 
ing colonies  and  in  methods  of  reproduction. 

Pleurococcus.  —  Pleurococcus  {Fig.  262),  often  called  Protococ- 
cus,  is  the  simplest  plant  of  the  group,  and  may  be  regarded  as 


Fig.  262. — Pleurococcus  vul- 
garis. Above,  a  single  plant 
consisting  of  a  single  cell  with  a 
definite  wall,  well  defined  nucleus, 
and  large  lobed  chloroplast;  be- 
low, left,  plants  dividing;  and 
below,  right,  a  group  of  four 
separate  plants.  X  540.  After 
Strasburger. 


308 


THALLOPHYTES 


one  of  the  simplest  of  the  Green  Algae.  It  forms  green  coatings, 
resembling  green  paint,  on  flower  pots,  damp  earth  or  walls,  and 
on  the  trunks  of  trees.  It  is  a  single,  globu- 
lar, non-motile  cell.  It  has  a  definite  w^all, 
a  large  lobed  chloroplast  suggesting  several 
chloroplasts,  and  its  nucleus  and  cytoplasm 
are  well  defined.  It  reproduces  entirely  by 
cell  division,  thus  forming  no  zoospores  or 
gametes.  They  are  small  plants  and  a  mass 
of  them  perceptil)le  to  the  eye  consists  of 
numerous  individuals.  They  divide  rapidly 
when  conditions  are  favorable,  and  daughter 
cells  recently  formed  and  not  yet  separated 
are  usually  seen  when  a  mass  of  individuals 
is  observed  with  the  microscope. 

Scenedesmus.  —  This    form,    which    is 
common  in  fresh  water,  is  often  classed  with 
the  Protococcales.      The  individuals  form 
simple  colonies  with  the  individuals  usually 
arranged  in  a  row  as  shown  in  Figure  263.     There  are  no  zoospores 


Fig.  263.  —  Scene- 
desmus. Above,  a 
colony  of  Scenedesmus 
quadricanda  consisting 
of  four  cells  arranged 
in  a  row;  below,  a  cell 
of  the  old  colony  form- 
ing a  new  colony. 
X  600.  Drawn  from 
West. 


Fig.  264.  —  Pediaslrum  horyanum.  At  the  left,  the  plate-like  colony  of 
cells,  some  of  which  have  formed  zoospores  and  from  one  of  which  the 
zoospores  are  escaping;  at  the  right,  zoospores  arranging  themselves  into 
a  new  colony.       X  about  400.     From  Braun. 


or  gametes,  but  reproduction  is  effected  by  the  division  of  each 
cell  into  daughter  cells  which  escape  as  a  new  colony. 


HYDRODICTYON 


309 


Pediastrum.  —  A  more  complicated  colony  occurs  in  Pedi- 
astrum  {Fig.  264),  another  form  common  in  ponds  and  other  quiet 
waters  in  warm  weather.  The  cells,  which  are  quite  numerous 
in  some  species,  form  plate-like  colonies  in  which  marginal  cells 
differ  in  form  from  those  within. 

Both  zoospores  and  gametes  are  produced  in  this  form.  Any 
cell  may  form  zoospores,  which  escape  from  the  mother  cell 
enclosed  in  a  membrane  and  then  arrange  themselves  into  a  new 
colony.     Instead  of  zoospores  the  cells  may  form  gametes,  which 


-v.t^^^ 


Fig.  265.  — •  Water-net,  Hydrodictyon  reticulatum.  a,  portion  of  a  net 
(X  about  2);  6,  a  cell  which  has  formed  zo6spore.s;  c,  the  zoospores  formed 
into  a  small  net  within  the  mother  cell;  d,  a  cell  in  which  gametes  have 
formed;  at  the  left  of  the  opening  through  which  the  gametes  are  escaping 
two  gametes  are  shown  fusing. 

resemble  zoospores  but  are  smaller  and  more  numerous.  The 
gametes,  since  they  are  alike,  form  zygospores,  and  each  zygo- 
spore upon  germination  produces  a  new  colony. 

Hydrodictyon.  —  This  is  the  remarkable  Water-net,  in  which 
the  cylindrical  colonies,  often  a  yard  or  more  in  length,  comprise 
thousands  of  cells  so  joined  as  to  enclose  polygonal  meshes  and 
thus  form  a  net  as  Figure  265  shows.  These  massive  colonies, 
buoyed  up  by  bubbles  of  oxygen  caught  within  them,  often  form 
extensive  floating  mats  in  lakes,  ponds,  and  sluggish  streams. 

New  nets  may  arise  from  zoospores  or  from  zygospores.  When 
a  cell  reaches  a  certain  size  and  other  conditions  are  right,  its 
protoplast  divides  itito  thousands  of  zoospores.  These  zoospores 
do  not  escape  but,  after  swimming  about  for  a  time  in  the  mother 


310 


THALLOPHYTES 


produced  within  a  cell. 


cell,  they  so  arrange  themselves  and  grow  together  at  points  of 
contact  as  to  form  a  miniature  net.  Through  the  softening  and 
decay  of  the  wall  of  the  mother  cell,  the  small  net  is  set  free 
and  by  the  mere  enlargement  of  its  cells  becomes  a  colony  of 
adult  size.  The  gametes  are  isogamous  and  are  formed  in  great 
numbers  by  certain  cells.  As  many  as  100,000  of  them  may  be 
Almost  as  soon  as  formed  they  escape 
from  the  mother  cell  and  begin  to  pair 
and  fuse.  The  zygospore  produces 
zoospores  which  at  first  pass  into  a  rest- 
ing stage  and  later  from  new  nets. 

Thus  in  the  Protococcales  the  individ- 
uals may  remain  separate  or  form  colo- 
nies which  are  exceedingly  complex  in 
the  higher  forms.  In  the  simplest  forms, 
as  Pleurococcus  illustrates,  reproduction 
is  by  cell  division  in  which  the  parent 
divides  to  form  two  new  plants,  but  in 
the  higher  forms  there  is  reproduction 
by  zoospores  and  isogametes.  Since 
their  sexuality  does  not  reach  the  heter- 
ogamous  condition,  they  are  not  so  ad- 
vanced in  this  respect  as  the  Volvocales 
are,  but  they  lack  motility  and  this 
feature  is  characteristic  of  the  higher 
plants,  which  are  adapted  to  live  on  land 
rather  than  in  the  water. 
Confervoid  Algae  (Confervales). — ^  The  Confervales  or  Con- 
fervoid  Algae  are  among  the  most  familiar  of  the  Green  Algae. 
Their  plant  bodies  are  usually  filaments,  commonly  consisting 
of  much  elongated  cyhndrical  cells  closely  joined  end  to  end  in 
a  single  row.  The  filaments  may  be  several  inches  in  length 
and  in  some  forms  much  branched.  In  a  few  forms  the  plant 
body  is  plate-like  instead  of  filamentous,  as  the  Sea  Lettuce  illus- 
trates (Fig.  266).  The  Confervales  are  common  in  lakes,  ponds, 
streams,  and  water  troughs,  where  many  of  them  grow  attached 
and  form  green  hair-like  fringes  about  rocks  and  other  ob- 
jects. More  than  700  species  of  them  are  known,  and  there 
is  considerable  variation  in  plant  body  and  methods  of  repro- 
duction. 


Fig.  266.  —  Sea  Lettuce 
(Ulva),  a  Confervoid  Alga 
having  a  plate-like  plant 
body.  This  plate-like  plant 
body  is  two  layers  of  cells 
in  thickness,  bright  green, 
and  resembles  a  leaf  in 
form.  Natural  size.  Re- 
drawn from  Bessey. 


ULOTHRIX 


311 


Ulothrix.  —  Ulothrix  (Fig.  267)  is  one  of  the  simpler  forms  of 
the  group,  and  its  filaments,  an  inch  or  two  in  length,  form  bright 
green  fringes  about  stones  and  other  objects  in  lakes,  ponds, 
streams,  and  troughs.  There  is  some  difTerentiation  within  the 
filament,  for  the  basal  cell  is  modified  into  a  holdfast  by  which 
the  filament  is  attached  to  a  support.  The  other  cells  arc  alike 
and  each  contains  one  nucleus  and  a  large  encircHng  chloroplast. 


Fig.  267.  —  Ulothrix  zonata.  A,  portion  of  a  filament,  showing  the  hold- 
fast and  a  number  of  vegetative  cells;  B,  portion  of  a  filament,  show- 
ing three  cells  containing  gametes;  C,  a  portion  of  a  filament,  showing 
gametes  escaping  at  b,  and  zoospores  formed  at  a  and  escaping  at  c;  D,  a  new 
filament  developing  from  a  zoospore,  the  character  of  which  is  shown  at  z. 
At  g  gametes  are  shown  fusing  to  form  zygospores.  At  zy  a  zygospore,  just 
after  the  fusion  of  the  gametes  and  when  fully  mature,  is  shown.  A  zygo- 
spore which  has  germinated  and  produced  four  zoospores  is  shown  at  y. 
X  200-300.     Redrawn  with  modification  from  Coulter  and  from  Strasburger. 

The  plant  reproduces  asexually  by  four-ciliate  zoospores,  and 
sexually  by  two-ciliate  isogametes.  The  zoospores  are  formed  usu- 
ally two  or  more  m  a  cell.  They  escape  together  from  the  mother 
cell  enclosed  in  a  membrane,  but  soon  separate  and  after  swim- 
ming about  for  a  short  tmie  become  attached  to  some  object  by 
the  ciliated  end  and  by  growth  and  cell  division  become  new 
filaments.  Some  cells  produce  gametes,  which,  besides  having 
only  two  cilia,  are  much  smaller  and  more  numerous  than  zo- 
ospores. After  escaping,  the  gametes  fuse  in  pairs  to  form  resting 
zygospores.     Upon  germination,  the   zygospore  does    not   pro- 


312 


THALLOPHYTES 


duce  ji  new  plant  directly  but,  as  in  H^^drodictyon,  produces 
a  number  of  zoospores  each  of  which  produces  a  new  plant. 
Thus,  instead  of  one,  a  number  of  new  plants  arise  from  the 
zygospore,  a  feature  of  advantage  in  the  multiplication  of  new 
plants. 

Another  form,  similar  in  a  number  of  waj^s  to  Ulothrix,  is 

Cladophora  which  has  long 
branched  filaments  that 
form  long,  green,  hair-like 
tufts,  which,  with  one  end 
anchored  to  a  stone  or  some 
other  object,  wave  back  and 
forth  in  moving  streams. 
The  cells  are  multinucleate 
and  contain  many  chloro- 
plasts.  Reproduction  is  by 
zoospores  and  isogametes, 
but  .the  zygospore  develops 
a  new  plant  directlj^ 

Oedogonium. — This  form 
(Fig.  268),  common  in  lakes 
and  ponds,  is  similar  to  Ulo- 
thrix in  the  character  of  the 
filament,  but  shows  marked 
advancement  in  methods  of 
reproduction.  The  z  o  - 
ospores,  formed  only  one  in 
a  cell  and  consequently  very 
large,  have  numerous  cilia 
forming  a  crown  at  the  for- 
ward  end.  Sexual  reproduc- 
tion is  distinctly  heteroga- 
mous.  The  eggs,  which  are 
large  and  packed  with  food, 
are  borne  in  much  enlarged 
cells  called  oogonia.  Each  oogonium  bears  one  egg  and  is  simply 
a  transformed  vegetative  cell  of  the  filament.  Other  small  cells 
produce  the  sperms  which  resemble  the  zoospores  except  in  size. 
The  sperms  swim  to  the  oogonia,  enter,  and  ff^rtilize  the  eggs  and 
thick-walled  resting  oospores  are  then  formed.     Upon  germina- 


FiG.  268.  —  Oedogonium.  A ,  a  portion 
of  a  filament  of  Oedogonium  echinosper- 
mum,  showing  some  vegetative  cells  and 
oogonium  above  and  some  antheridia  be- 
low from  which  sperms  are  escaping;  B,  a 
portion  of  a  female  filament  of  OEDOGO- 
NIUM HUNTIT,  showing  oogonia  and 
two  dwarf  male  plants  attached  near  the 
oogonia;  C,  zoospores  of  an  Oedogonium 
escaping  from  the  cells  of  the  filament. 
X  about  300.     Drawn  from  Wolle. 


COLEOCHAETE 


313 


tion  the  oospore  forms  four  zoospores,  each  of  which  develops  a 
new  filaiiient.  In  some  forms  of  Oedogonium  tliere  are  both 
male  and  female  filaments.  In  some  species  the  male  plants  are 
miniature  filaments  and  attach  themselves  to  the  female  plants, 
where  they  produce  sperms  in  their  terminal  cells. 

Coleochaete.  —  This  form  (Fig.  269),  found  growing  attached 
to  water  plants,  has  a  disk-shaped  plant  body  and  also  presents 
some  new  features  in  connection  with  its  reproduction.  Like 
Oedogonium  it  reproduces  by  zoospores  and  sexually  by  oospores. 
One  of  the  new  features  is 
the  development  of  a  case 
around  the  oospore  by  the 
adjacent  cells.  This  fea- 
ture suggests  a  close  rela- 
tionship of  this  form  to  the 
higher  Algae,  where  the 
formation  of  a  case  around 
the  immediate  product  of 
the  oospore  is  a  prevalent 
feature.  The  second  new 
feature  is  that  the  oospore 
upon  germination  develops 
neither  a  plant  nor  zo- 
ospores, but  a  structure 
consisting  of  several  cells 
each  of  which  develops  a 
zoospore  from  which  a  new 
plant  arises.  Thus  between 
fertilization    and    the    de- 


FiG.  2m.  — Coleochaete  scutata.  A,  the 
plate-like  plant  body  with  two  oogonia 
developed  (X  25) ;  B,  thick-walled  oospore 
surrounded  by  vegetative  cells  (much  en- 
larged) ;  C,  a  much  enlarged  section  through 
the  oospore  and  its  jacket  of  sterile  cells, 
showing  the  multicellular  body  produced 
by  the  oospore,  each  cell  of  which  pro- 
duces a  zoospore.  Redrawn  from  WoUe, 
Atkinson,  and  Altmanns. 
velopment   of  new   plants, 

there  is  introduced  a  new  structural  stage  and  one  that  is  char- 
acteristic of  higher  plants.  These  new  features  with  others  have 
led  to  the  theory  that  the  higher  plants  have  evolved  from  Algae 
of  the  type  of  Coleochaete. 

In  having  multicellular  plant  bodies  and  more  advanced 
methods  of  reproduction,  the  Confervales,  as  a  group,  show 
advancement  over  the  preceding  groups.  The  plant  body  is  a 
simple  filament,  branched  filament,  or  a  disk-shaped  structure. 
Sexual  reproduction,  which  is  isogamous  in  the  lower  forms, 
advances  to  heterogamy  where  the  two  kinds  of  gametes  occur  in 


314 


THALLOPHYTES 


special  cells  and  often  on  different  plants.  Also  in  the  higher 
forms,  the  introduction  of  a  case  around  the  oospore,  and  a  new- 
structural  stage  between  fertilization  and  the  formation  of  new 
plants,  suggests  a  relationship  to  the  higher  plants.  On  the  other 
hand,  the  simpler  forms  resemble  some  of  the  Protococcales  from 
w^hich  the  Confervales  have  probably  been  evolved. 

Conjugating  Algae  (Conjugales).  —  This  group  is  so  named 
because  of  the  peculiar  conjugating  habit,  in  which  the  contents 
of  two  cells  fuse  to  form  zygospores.  Some  are  unicellular  but 
many  are  filamentous.     They  include  Spirogyra  and  others  that 


Fig.  270.  — -  Desmids.  a  and  b,  two  ccmmon  species  of  Desmids  highly 
magnified;  at  the  right  of  c,  a  Desmid  dividing,  and  at  the  left  of  c,  each 
daughter  cell  resulting  from  the  division  developing  a  new  half;  at  d,  the  pro- 
toplasts of  two  Desmids  are  escaping  and  conjugating.     Redrawn  from  Curtis. 

are  very  common  nearly  everywhere  in  fresh  water.  They  are 
free  floating,  and  the  filamentous  forms  often  form  extensive 
floating  mats,  which  are  buoyed  up  by  the  oxygen  entangled 
among  the  filaments.  Some,  owing  to  the  shape  and  arrange- 
ment of  their  chloroplasts,  are  attractive  plants  under  the  micro- 
scope. One  peculiar  feature  of  the  group  is  that,  although  the 
plants  are  aquatic,  there  are  no  ciliated  cells  of  any  kind. 

Desmids.  —  The  simplest  of  the  Conjugales  are  the  Desmids, 
which  are  unicellular  floating  plants  that  exhibit  a  variety  of 
shapes  and  some  are  extremely  beautiful  (Fig.  270).     They  are 


SPIROGYRA 


315 


abundant  fresh  water  plants,  and  in  the  examination  of  other 
forms  of  fresh  water  Algae  with  the  microscope  one  usually  finds 
some  Desmids  present.  The  cell  is  peculiar  in  being  organized 
into  symmetrical  halves,  which  are  separated  by  a  constriction 
that  forms  an  isthmus.  The  nucleus  is  in  the  isthmus,  and  in 
each  half  there  is  a  chloroplast  and  a  number  of  pyrenoids. 

They  reproduce  in  two  ways,  by  cell  division  and  by  zygo- 
spores. In  multiplying  by  cell 
division,  the  cell  divides  at  the 
isthmus,  the  halves  separate,  and 
the  portion  of  the  isthmus  re- 
maining to  each  half  develops  a 
new  half  and  thus  a  new  individ- 
ual is  formed.  In  sexual  repro- 
duction the  cells  pair  and  the 
protoplasts,  which  escape  through 
ruptures  at  the  isthmus,  fuse  and 
form  a  zygospore.  Sometimes 
the  cells  after  pairing  become  con- 
nected by  a  tube  through  which 
the  protoplasts  reach  each  other. 
In  either  case  the  entire  proto- 
plasts of  cells  conjugate. 

Spirogyra,  —  Spirogyra  {Fig. 
271),  very  common  in  ponds, 
sluggish  streams,  and  watering 
troughs,  is  the  most  familiar  fila- 
mentous form  of  the  Conjugales 
and  the  one  most  commonly 
studied  in  elementary  classes.  It 
gets  its  name  from  its  large  and 
beautiful  spiral  chloroplasts.  Its 
cells  are  all  alike  and  it  pro- 
duces no  zoospores.  Its  sexual  reproduction,  in  which  the  gam- 
etes reach  each  other  through  tubes,  is  its  important  feature. 
Under  certain  conditions,  filaments  pair  and  line  up  side  by  side. 
In  this  position,  the  cells  of  the  filaments  grow  toward  each 
other  in  tubular  projections  which  unite  and  form  open  passage 
ways  between  the  cells  of  the  paired  filaments.  The  protoplasts 
of  one  filament  pass  through  these  tubes  and  fuse  with  the  pro- 


A 

Fig.  271.  —  A  species  of  Spiro- 
gyra. A,  a  portion  of  a  filament 
showing  a  vegetative  cell  with  its 
spiral  chloroplasts  (c)  and  nucleus 
(/()  (X  100);  B,  filaments  conju- 
gating and  two  zygospores  (z) 
fully  formed;  C,  a  zygospore 
germinating  and  producing  a  new 
filament  (X  150).  A  and  5  from 
nature,  and  C  from  WoUe. 


316 


THALLOPHYTES 


toplasts  of  the  other  fihiment  as  shown  in  Figure  271.  The 
protoplasts  are  unhke,  for  one  migrates  while  the  other  does 
not.  In  behavior  the  migrating  protoplasts  may  be  regarded 
as  sperms  and  the  passive  ones  as  eggs,  although  they  show  no 
differentiation  in  size  or  structure.  Also,  the  filament  which 
loses  its  protoplasts  may  be  regarded  as  male  and  the  receiving 
filament  as  a  female  individual.  The  zygospore  builds  about 
itself  a  heavy  wall  and  at  the  end  of  a  rest  period  develops 
directly  into  a  new  filament. 

It  is  now  seen  that  the  Conjugales  stand  quite  apart  from  the 
previous  groups  in  having  no  zoospores  or  swimming  gametes. 


Fig.  272.  —  A  species  of  Vaucheria  {Vaucheria  sessilis),  showing  the 
coenocytic  habit  of  the  filament,  the  oogonia  at  o,  the  antheridia  at  a,  and  the 
sperms  escaping  from  an  antheridium  and  entering  an  oogonium  at  s.  X  75. 
Partly  drawn  from  nature  and  partly  diagrammatic. 


and  in  having  a  peculiar  kind  of  conjugation,  in  which  entire 
protoplasts  fuse  and  commonly  reach  each  other  through  tubes. 
Although  the  gametes  are  alike  in  size  and  structure,  they  show 
some  differentiation  in  the  way  they  behave.  The  group  is 
considered  a  highly  specialized  one. 

Tubular  Algae  (Siphonales).  —  These  Algae,  of  which  there  are 
about  300  species,  are  so  named  because  the  plant  body,  no  matter 
how  long  and  thread-like,  has  no  cross  walls  and,  therefore, 
resembles  a  tube  filled  with  protoplasm.  The  protoplasm  con- 
tains many  nuclei  and  many  chloroplasts,  and  may  be  regarded 
as  a  much  elongated  multinucleate  cell  or  as  a  filament  with  cross 
walls  omitted.  Such  a  plant  body  is  called  a  coenocyte.  The 
majority  of  the  Siphonales  are  marine  forms,  living  in  warm  seas, 


VAUCHERIA 


317 


but  there  are  a  number  of  species  living  in  fresh  water  and  some 
on  moist  shaded  soil. 

Vaucheria.  —  Vaucheria  {Fig.  272)  is  one  form  of  the  Sipho- 
nales  that  is  common  in  fresh  water  and  on  moist  shaded  soil. 
The  long  filaments,  usually  much  coarser  than  those  of  Spiro- 
gyra  and  usually  branched,  interlace  and  form  felt-like  masses, 
on  which  account  Vaucheria  is  often  called  Green  Felt.  The 
green  or  yellowish  green  felt- 
like mats  of  the  species  grow-  o.t"./ 
ing  on  moist  soil  are  common 
in  flowerpots  and  on  and  under 
the  benches  in  greenhouses. 
Other  species  are  common  in 
ponds  and  sluggish  streams. 

Vaucheria  forms  zoospores 
and  heterogametes.  In  form- 
ing a  zoospore  a  portion  of  pro- 
toplasm at  the  end  of  the  fila- 
ment is  cut  off  from  the  rest 
by  a  cross  wall.  This  severed 
mass  of  protoplasm  escapes 
from  the  filament  as  a  multi- 
nucleate and  multiciliate  zo- 
ospore, large  enough  to  be  seen 
with  the  naked  eye.  After 
swimming  about  for  a  time 
the  zoospore  comes  to  rest  and 
elongates  into  a  new  filament. 

Sexual  reproduction  shows 
advancement  in  that  the 
gametes  are  borne  in  well-de- 
fined  sex   organs,  which   are 

special  structures  for  bearing  sex  cells.  The  oogonium,  oval  in 
shape,  bears  one  large  egg,  and  the  antheridium  containing  many 
sperms  is  near  it  and  is  the  end  cell  of  a  short  curved  branch.  The 
sperms  escape,  reach  the  egg  through  a  special  opening  in  the 
oogonium,  and  one  of  them  fertilizes  the  egg.  The  heavy-walled 
oospore  upon  germination  forms  a  new  filament  directly. 

There  are,  however,  some  Siphonales  in  which  sexual  reproduc- 
tion is  of  a  simpler  type.     For  example,  in  Botrydium  (Fig.  273), 


Fig.  273.  —  Botrydium  granulatum. 
At  the  left,  the  vegetative  plant  body, 
showing  the  root-like  projections  be- 
low and  the  balloon-like  top  above 
ground;  at  the  right,  a  plant  in  which 
zoospores  have  formed  and  are  escap- 
ing; between  the  enlarged  plants, 
plants  about  natural  size.  Draw^^  with 
modifications  from  WoUe. 


318  THALLOPHYTES 

a  form  with  a  small  balloon-shaped  plant  body,  which  is  commonly 
found  projecting  from  moist  soil,  there  are  no  sex  organs  and  the 
gametes  are  aUke. 

The  Siphonales  are  most  peculiar  in  having  a  tube-like  plant 
body.  In  the  production  of  well-defined  sex  organs  they  show 
considerable  advancement  in  sexual  reproduction. 

Summary  of  Green  Algae.  —  In  having  chloroplasts  and  well- 
defined  nucleus  and  cytoplasm,  the  cells  of  the  Green  Algae  are 
more  advanced  than  those  of  the  Blue-green  Algae.  In  the 
simplest  forms  the  plant  body  is  a  single  cell,  either  motile  or 
non-motile,  and  the  uniting  of  cells  into  colonies  is  a  prominent 
feature.  In  the  higher  forms  the  plant  body  is  multicellular  and 
in  the  form  of  a  filament  or  disk.  The  multicellular  forms  have 
not  only  established  the  habit  of  cells  living  together  to  form  a 
complex  plant  body  but  have  also  to  some  extent  differentiated 
cells.  These  habits  look  forward  toward  the  formation  of  plants 
consisting  of  a  countless  number  of  cells,  which  are  differentiated 
into  tissues,  such  as  occur  in  the  higher  plants.  They  introduced 
sex  cells  which  were  at  first  alike,  and  later  differentiated  the  sex 
cells,  thus  introducing  eggs  and  sperms,  the  sex  cells  of  the  higher 
plants.  They  also  introduced  sex  organs  which  are  prominent 
structures  in  the  Mosses  and  Ferns. 


Brown  Algae  (Phaeophyceae) 

These  Algae  are  marine  forms,  occurring  on  all  sea  coasts  but 
more  abundantly  in  the  cooler  waters.  They  have  two  pigments, 
chlorophyll  and  a  brown  pigment  called  fucoxanthin,  but  the 
brown  pigment  obscures  the  green  one  and  determines  the  color 
of  the  plant.  Both  pigments  probably  help  in  the  manufacture 
of  food,  although  the  exact  function  of  fucoxanthin  is  not  known. 
These  Algae  grow  anchored  by  holdfasts  to  the  rocks  and  their 
bodies  are  so  tough  and  leathery  that  they  are  not  injured  by  the 
beating  of  the  waves. 

Although  a  few  are  simple  filaments,  most  of  them  are  much 
more  complex  than  any  of  the  Green  Algae.  The  plant  body  of 
the  majority  of  them  not  only  consists  of  a  greater  number  of 
cells,  but  there  is  more  differentiation  among  the  cells  than  in  the 
Green  Algae.  In  the  largest  of  them  the  plant  body  often  attains 
a  length  of  several  hundred  feet.     As  shown  in  Figure  274,  the 


LAM  IN  ARIAS 


319 


plant  body  coniinonly  consists  of  a  stalk  bearing  leaf-like  branches 
and  attached  to  a  support  by  root-like  holdfasts.  One  might 
think  such  a  plant  too  complex  to  be  classed  as  a  Thallophyte, 
for,  according  to  definition,  a  Thallophyte  is  a  plant  not  differen- 
tiated into  roots,  stem,  and  leaves.  However,  when  the  struc- 
ture of  these  parts  that  so  much  resemble  roots,  stems,  and  leaves 
is  studied,  one  finds  that  they  are  too  simple  in  structure  to  be 
classed  as  such  organs,  although  they  mark  a  notable  advance- 
ment over  the  Green  Algae  in  the  differentiation  of  the  plant  body. 
Some  have  special  swollen  regions  called  air  bladders,  which  help 


Fig.  274.  —  One  of  the  Brown  Algae,  Macrocystis,  showing  the  root-Uke 
hoklfasts,  the  stem-like  axis,  and  the  leaf-Uke  blades.  Much  reduced.  Re- 
drawn with  modifications  from  Harvey. 


the  plant  to  float,  and  in  connection  with  reproduction  there  is 
much  differentiation  shown  by  some  forms. 

There  are  about  1000  species  of  Brown  Algae  known  and  these 
are  divided  into  two  groups,  one  of  which  comprises  the  Kelps 
and  closely  related  forms,  and  the  other,  the  Rockweeds  and  Gulf- 
weeds. 

Kelps  and  Closely  Related  Forms  (Phaeosporales).  —  This 
order  comprises  a  number  of  families  of  which  tlie  Laminarias  or 
Kelps  are  the  largest  forms. 

Laminarias.  —  These  are  the  largest  of  Algae,  and  include  such 
conspicuous  forms  as  Nereocystis,  Postelsia,  or  Sea  Palm,  and 
the  huge  Macrocystis  (Fig.  274),  which  is  sometimes  more  than 
200  feet  in  length.     It  is  from  the  massive  plant  bodies  of  the 


320 


THALLOPHYTES 


Kelps  that  much  potassium  for  fertihzers  is  obtained  (Fig.  275). 
Although  the  Kelps  have  massive  and  complex  plant  bodies, 
their  reproduction,  so  far  as  known,  is  not  so  complete  as  that  of 
some  Green  Algae.  Their  reproduction  is  sexual  and  the  small 
ciliated  gametes  are  borne  in  special  cells  which  occur  in  patches 


Fig.  275.  —  Harvesting   Kelp   on  the   Pacific   Coast.     From  Report   100, 
U.  S.  Dept.  of  Agriculture. 

on  the  surfaces  of  the  leaf-like  branches.  The  zygospore  develops 
a  new  plant  directly. 

Ectocarpus.  —  This  form  (Fig.  276),  although  belonging  to  the 
same  order,  contrasts  strikingly  in  size  with  the  Kelps,  for  it  is  a 
slender  filamentous  form  not  much  larger  than  some  of  the  Green 
Algae.  This  form  also  show^s  some  interesting  features  in  connec- 
tion with  reproduction  which  is  effected  through  the  production 
of  both  zoospores  and  gametes. 

The  zoospores  are  produced  in  certain  cells  which  become  trans- 
formed into  sporangia.  In  forming  a  sporangium,  a  single  cell 
within  the  filament  or  at  the  end  of  a  branch  usually  enlarges 
and  its  protoplasm  divides  up  into  zoospores.  The  zoospores 
bear  their  cilia  laterally  and  not  terminally  as  in  the  Green 
Algae,  but  function  in  the  same  way  by  growing  directly  into 
new  plants. 


ROCKWEEDS  AND   GULFWEEDS   (FUG ALES) 


321 


The  gametes  arc  produced  in  a  iimlticcllular  structure  known 
as  a  gametamjium,  and,  as  in  case  of  a  sporangium,  the  gametan- 
gium  may  be  formed  from  a  cell  within  the  filament  or  from  a 
terminal  cell  on  a  short  lateral  branch.  The  small  cubical  cells 
composing  a  gametangium  are  packed  closely  together  and  each 


Fig.  276.  —  Portions  of  two  fila- 
ments of  Edocarpus,  showing  repro- 
duction. At  s  are  shown  spo- 
rangia, and  between  the  filaments, 
a  zoospore.  At  g  are  shown  a  game- 
tangium and  a  single  sperm  and  two 
sperms  fusing  at  the  right .  Redrawn 
with  modifications  from  Gurtis. 


Fig.  277.  —  Plant  body  of 
Fucus'  vesiculosus  (X  I).  In 
the  swollen  tips  are  the  con- 
ceptacles  in  which  the  sex 
organs  occur,  and  at  various 
places  occur  bladder  -  Uke 
floats. 


produces  a  biciliate  zoospore-like  gamete.  After  escaping  the 
gametes  fuse  in  pairs  and  form  zygospores.  In  this  plant  such 
gradations  occur  between  sporangia  and  gametangia  and  between 
zoospores  and  gametes,  as  to  afford  considerable  support  for  the 
theory  that  gametes  are  simply  zoospores  which  are  too  small 
to  function  alone. 

Rockweeds   and   Gulf  weeds    (Fucales).  —  In   this  order  the 
plant  body  may  reach  a  meter  in  length,  but  is  usually  much 


322 


THALLOPHYTES 


smaller.     Although  strictly  nquatic,  they  produce  no  zoospores 
and  their  sexual  reproduction  is  much  specialized. 

Rockweeds.  —  These  are  very  common  Seaweeds  and  are 
especially  abundant  on  rocky  shores.  The  plant  body,  sometimes 
a  foot  or  more  in  length,  is  much  branched  and  has  bladder- 
like floats  and  commonly  special  reproductive  structures.  The 
Rockweeds  are  common  in  fish  markets,  being  used  as  a  packing 


Fig.  278.  —  Reproduction  in  Fucits  vesiculofius.  a,  section  through  a 
swollen  tip,  showing  sections  through  some  of  the  conceptacles;  b,  much 
enlarged  section  through  an  oogonial  conceptacle,  showing  the  pore-like  open- 
ing to  the  exterior  and  the  oogonia  within;  c,  a  similar  section  through  a 
conceptacle  containing  antheridia  which  appear  as  small  bodies  on  the  fila- 
ments projecting  from  the  walls  of  the  conceptacle;  d,  antheridia  much  en- 
larged and  one  antheridium  shedding  its  sperms;  e,  oogonium  from  which 
the  eggs  are  escaping;  /,  sperms  swarming  around  an  egg. 


in  the  shipment  of  crabs  and  other  shell  fish.  Along  the  west 
coast  of  South  America  and  also  in  other  countries,  Fucus  is  used 
for  food  by  the  inhabitants,  and  it  is  also  used  as  a  fertilizer  and 
as  a  source  of  iodine. 

Fucus  vesiculosus,  one  of  the  commonest  of  the  Rockweeds,  will 
serve  to  illustrate  the  character  of  the  plant  body  and  the  peculiar 
features  of  reproduction,  the  former  being  shown  in  Figure  277 
and  the  latter  in  Figure  278.     The  gametes  are  differentiated 


GULFWEEDS   (SARGASSUM) 


323 


and  are  ])orno  in  sex  organs,  whicli  are  also  (juitc  unlike.  Tlie  s(>x 
organs  are  borne  in  hollow  eonceptacles,  wiiicli  occur  in  large 
numbers  in  the  swollen  tips  of  the  branches.  Each  conceptacle 
opens  to  the  exterior  by  a  pore-like  opening.  In  this  species  the 
male  and  female  sex  organs  do  not  occur  together  in  the  same 
eonceptacles  as  they  do  in  some  species.  The  oogonium  is  a 
large,  globular,  stalked  cell  and  in  this  species  contains  eight  eggs, 
but  the  number  ranges  from  one  to  eight  in  other  species  of 
Fucus.  The  antheridia  are  borne 
on  the  lateral  branches  of  much- 
branched  filaments,  which  pro- 
ject from  the  wall  of  the  concep- 
tacle. They  are  oval  cells  which 
produce  numerous  sperms.  A 
curious  feature  of  Fucus  is  that 
the  eggs  as  well  as  the  sperms  are 
discharged  from  the  conceptacle 
before  fertilization.  The  eggs 
while  passively  floating  about 
are  surrounded  by  swarms  of 
spcTms,  which  sometimes,  by  their 
vigorous  movements,  give  the 
eggs  a  rotary  motion.  In  fertil- 
ization one  sperm,  after  pene- 
trating the  cytoplasm  of  the  egg, 
fuses  with  the  egg  nucleus  and 
thus  an  oospore  is  formed  which 
develops  a  new  plant. 

Gulfweeds  (Sargassum). — 
The  Gulfweeds,  well  known  in 
connection  with  the  Sargasso  Sea,  are  sometimes  a  meter  in 
length  and  are  more  differentiated  than  the  Rockweeds  (Fig. 
279).  In  form  the  stalks  and  leaf-like  branches  resemble  very 
much  the  stems  and  leaves  of  the  higher  plants,  although  they 
are  very  different  in  structure.  The  stems  are  at  first  anchored 
by  root-like  holdfasts  and  bear  many  stalked  air  bladders,  which 
buoy  up  the  plant  when  attached  and  float  it  when  torn  free. 
Other  short,  thick,  axillary  branches  contain  the  eonceptacles. 
So  far  as  known  their  reproduction  is  similar  to  that  of  the  Rock- 
weeds. 


Fig.  279.  —  A  portion  of  a  plant 
of  Sargassum  vulcjcire,  showing  the 
floats  and  the  stem-  and  leaf-like 
structures.     X  h- 


324  THALLOPHYTES 

It  is  now  evident  that  among  the  Brown  Algae  there  is  more 
differentiation  of  plant  body  and  more  specialization  in  sexuality 
than  among  the  Green  Algae.  Except  in  their  lowest  forms,  they 
show  no  affinities  with  the  Green  Algae  and  consequently  are 
supposed  to  have  originated  independently  and  probably  from 
such  organisms  as  gave  rise  to  the  Green  Algae.  Unlike  the  Green 
Algae  they  show  no  evidence  of  having  led  to  higher  forms. 

Red  Algae  (Rhodophyceae) 

These  Algae  are  mainly  marine  forms,  although  some  forms 
occur  in  streams.  Besides  the  green  they  have  a  red  pigment 
called  pMjcoerythrin  which  determines  their  color.     Some  also 


Fig.  280.  —  A  finely  branched  Red  Alga.      Natural  size. 

have  a  blue  pigment,  phycocyanin.  They  are  not  so  bulky  as  the 
Brown  Algae,  but  they  exceed  them  in  number  of  species  and  are 
much  more  diversified  in  form.  Some  are  mere  filaments  no 
larger  than  Vaucheria.  They  live  mostly  below  low  water  mark 
and  often  at  depths  of  more  than  100  feet  in  clear  waters. 

The  plant  body,  commonly  only  a  few  inches  in  length,  is  flat, 
thin,  and  flexible  and  usually  much  branched.  Some  kinds  are 
finely  branched,  as  the  Sea  Mosses  noted  also  for  their  beautiful 
colors  of  red,  violet,  and  purple  {Fig.  280).     As  in  the  Brown 


RED  ALGAE    (RHODOPHYCEAE)  325 

Algae,  the  plant  botly  is  commonl}^  differentiated  into  parts 
similar  in  form,  although  not  in  structure,  to  the  roots,  stems, 
and  leaves  of  the  higher  plants.  The  cells  are  commonly  ar- 
ranged in  such  definite  lines  that  the  plant  body  has  the  appear- 
ance of  a  bundle  of  closely  joined  simple  filaments.  The  evident 
protoplasmic  connections  between  cells  and  the  gelatinization  of 
cell  walls  are  other  notable  features. 


Fig.  281.  —  Irish  Moss,  Chondrus  crispus,  mucli  used  for  food.    Natural  size. 

The  life  history  of  some  of  them  is  quite  complex.  The}'  have 
spores,  but  their  spores  and  likewise  their  gametes  have  no  cilia, 
a  curious  feature  since  these  plants  are  wholly  aquatic.  The 
female  sex  organ  is  multicellular  and  more  complex  than  the  sex 
organs  of  the  Brown  Algae. 

Several  forms  of  the  Red  Algae  are  of  economic  importance. 
Some  are  used  as  food,  being  dried  and  kept  for  long  periods 
The  gelatinous  material  obtained  from  Red  Algae  forms  a  delicacy 


326 


THALLOPHYTES 


much  desired  by  some  people.  The  form  called  Irish  Moss, 
shown  ill  Figure  281,  is  collected  in  large  quantities  and  employed 
in  the  manufacture  of  jelly,  which  is  used  directly  as  food  and  as 
the  basis  for  the  preparation  of  other  foods.  Agar-agar,  which 
is  used  as  a  medium  in  which  Bacteria  and  Fungi  are  grown,  is  a 
gelatinous  product  obtained  from  Red  Algae. 

Nemalion.  —  This  plant  is  one  of  the  simpler  forms  of  Red 
Algae.     The  plant  body  is  a  rather  soft,  cord-like,  branching 


Fig.  282.  — Reproduction  in  Nemalion.  A,  a  portion  of  Nemalion,  bearing 
antheridia  at  a  and  at  the  right  a  procarp  consisting  of  carpogonium  (c)  and 
trichogyne  (0  to  the  tip  of  which  two  sperms  have  become  attached;  B, 
fertilized  carpogonium  beginning  to  develop  branches  on  the  ends  of  which 
the  carpospores  are  borne;  C,  mature  cystocarp  consisting  exteriorly  of  carpo- 
spores-.     X  100-150.     Redrawn  with  modifications  from  Bornet. 


structure,  composed  of  a  large  number  of  filaments,  which  are 
held  together  by  a  stiff  gelatinous  substance.  There  is  a  central 
axis  of  delicate  threads  and  an  outer  region  consisting  of  short 
branches  pointing  outward. 

Nemalion  produces  both  spores  and  gametes  {Fig.  282).  The 
two  kinds  of  reproduction  are  intimately  related,  for  the  produc- 
tion of  spores  follows  closely  as  a  result  of  fertilization. 

The  female  sex  organ,  which  is  a  very  peculiar  structure  in  the 
Red  Algae,  is  called  a  procarp.     In  Nemalion  the  procarp  is  borne 


POLYSIPHONIA  327 

on  the  end  of  a  branch  and  at  first  apparently  consists  of  two 
cells,  a  basal  one  called  carpogonium  and  a  much  elongated 
terminal  one  called  trichogyne.  The  two  cells  are  not  separated 
by  a  wall  and  the  nucleus  soon  disappears  from  the  trichogyne 
and  the  two  cells  then  appear  as  a  single  one  with  a  bulbous  base 
and  a  hair-like  extension.  The  carpogonium  corresponds  to  the 
oogonium  in  other  Algae,  for  it  contains  a  protoplast  which  func- 
tions as  an  egg. 

The  antheria,  which  are  borne  in  clusters  at  the  ends  of  short 
branches,  are  single  cells,  and  the  protoplast  of  each  antheridium 
becomes  binucleate  and  functions  as  a  sperm.  After  these 
binucleate  protoplasts  are  discharged  from  the  antheridia,  they 
depend  upon  water  currents  to  carry  them  to  the  female  sex 
organs  as  they  have  no  cilia.  When  they  come  in  contact  with 
the  trichogyne,  the  two  walls  in  contact  are  resorbed,  and  the  two 
male  nuclei  of  the  sperm  pass  into  the  trichogyne  through  the 
perforation.  A  number  of  sperms  may  discharge  their  nuclei 
into  the  same  trichogyne,  but  only  one  male  nucleus  passes  on 
into  the  carpogonium  and  fuses  with  the  female  nucleus.  After 
fertilization,  the  carpogonium  develops  numerous  short  filaments, 
each  of  which  bears  a  spore,  called  a  carpospore,  at  its  tip.  The 
carpospores,  short  filaments,  and  the  carpogonium  together  con- 
stitute the  structure  known  as  a  cystocarp.  The  carpospores 
upon  germination  develop  sexual  plants,  thus  completing  the  life 
histor3^ 

Polysiphonia.  —  This  plant  (Fig.  283)  is  a  representative  of 
the  complex  forms  of  Red  Algae.  It  is  a  much-branched  complex 
filament  and  is  so  named  because  it  has  a  central  row  of  elongated 
cells  (axial  siphon),  enclosed  by  peripheral  cells.  This  plant 
presents  much  differentiation  and  ordinarily  a  life  history  in- 
volves three  types  of  individuals  —  male,  female,  and  sexless 
plants. 

The  male  plants  bear  their  antheridia  on  very  short  lateral 
branches  which  arise  from  the  axial  siphon  and  bear  the  an- 
theridia somewhat  laterally  on  their  tips.  The  protoplast  of  an 
antheridium  contains  only  one  nucleus  and  is  not  discharged  as 
in  Nemalion,  but  the  antheridium  breaks  off  bodily  and  is  floated 
to  the  trichogyne. 

The  female  plant  produces  a  procarp  more  complex  than  that 
of  Nemalion.     The  procarp  consists  of  other  cells  in  addition  to 


328 


THALLOPHYTES 


the  carpogoniuni  and  trichogyne.  The  pericentral  cell,  the  large 
cell  of  the  axis  from  which  the  carpogoniuni  arises  and  the  vege- 
tative cells,  known  as  auxiliary  cells,  surrounding  the  carpogo- 
niuni take  part  in  forming  the  cystocarp  and  are  therefore  con- 
sidered a  part  of  the  procarp.     So  in  polysiphonia  a  procarp 


Fig.  283.  —  Polysiphonia  violacea.  A,  a  part  of  a  plant  showing  the  branch- 
ing and  multicellular  character  of  the  filament  (X  75);  B,  a  branch  bearing 
antheridia,  some  of  which  have  broken  away  (X  400);  C,  branch  bearing 
a  procarp  consisting  of  carpogonium  and  adjacent  cells  at  c  and  trichogyne 
(0  to  the  tip  of  which  a  sperm  is  attached  ( X  500) ;  D,  branch  bearing  a 
mature  cystocarp  (cy)  from  which  carpospores  are  shown  escaping  through 
an  opening  in  the  jacket  of  the  cystocarp  (X  75);  at  the  right  is  a  part 
of  a  tetrasporic  plant  bearing  three  tetrasporangia  (X  100). 

consists  of  trichogyne,  carpogonium,  pericentral  cell,  and  auxiliary 
cells. 

After  fertilization,  which  is  essentially  the  same  as  in  Nemalion, 
the  carpogonium,  pericentral  cell,  and  auxiliary  cells  unite  in 
forming  a  large  chamber  from  which  lobes  arise,  and  on  the  ends 
of  these  lobes  the  carpospores  are  produced.  In  the  meantime 
vegetative  cells,  growing  up  from  below,  form  a  jacket  which  en- 
closes the  spore-bearing  structure,  thus  completing  the  formation 
of  the  cystocarp  (meaning  a  fruit  case),  which  in  this  plant  is  a 
genuine  cystocarp.  From  this  cystocarp  the  carpospores  escape 
and  upon  germination  produce  an  asexual  or  tetrasporic  plant. 


FLAGELLATES  329 

The  tetrasporic  plant  in  character  of  phmt  body  is  very  simihir 
to  the  sex  plants.  On  it  no  sex  organs  occur.  It  bears  spores 
known  as  tetraspores,  so  named  because  the  number  occurring  in 
a  sporangium  is  four.  Why  the  plant  is  called  a  tetrasporic  plant 
is  now  clear.  The  sporangia,  which  have  a  one-celled  stalk,  arise 
laterally  from  the  axial  siphon  and  push  their  way  through  the 
peripheral  cells.  The  tetraspores  escape  and  upon  germination 
give  rise  to  plants  that  bear  sex  organs,  either  antheridia  or 
procarps. 

This  type  of  life  history  in  which  sexual  plants  alternate  with 
asexual  plants  is  a  feature  of  considerable  significance  because 
it  is  a  feature  characteristic  of  plants  above  Thallophytes.  It 
is  known  as  ''alternation  of  generations"  and  its  significance  will 
be  explained  in  the  groups  where  it  is  a  well-established  feature. 
The  alternation  of  generations,  the  cystocarp,  and  more  complex 
female  sex  organs  are  the  chief  features  introduced  by  the  Red 
Algae. 

When  Polysiphonia  is  compared  with  some  of  the  simplest 
forms  of  Algae,  as  some  of  the  one-celled  Blue-green  Algae  or  even 
Pleurococcus,  it  is  obvious  that  the  Algae  made  notable  advance- 
ments. The  plant  body,  a  single  cell  in  the  simplest  forms, 
becomes  a  multicellular  plant  body  showing  considerable  differ- 
entiation of  parts  as  to  form  and  function  in  the  higher  forms. 
Reproduction,  accomplished  entirely  by  cell  division  in  the 
simplest  Algae,  gradually  becomes  more  complex  through  the 
groups,  involving  zoospores,  gametes,  the  differentiation  of 
gametes,   and  the  development  of  sex  organs. 

There  is  very  little  reliable  data  as  to  how  each  group  of  Algae 
arose.  It  is  not  probable  that  they  arose  from  each  other,  but 
probably  all  have  developed  independently  from  some  unknown 
ancestor.  Regardless  of  how  they  arose,  the  groups  mark  in  a 
general  way  some  of  the  steps  in  the  evolution  of  the  higher 
plants. 

Some  Alga-like  Thallophytes  not  Definitely  Classified 

There  are  three  groups  of  alga-like  plants,  the  Flagellates,  Di- 
atoms, and  Stoneworts,  which  have  not  been  definitely  classified. 

Flagellates.  —  These  are  free-swimming  unicellular  organisms 
of  fresh  water.  They  have  both  plant  and  animal  features,  on 
which  account  they  are  regarded  as  intermediate  between  the 


330 


THALLOPHYTES 


plant  and  animal  kingdoms.  The  protoplast  is  naked  or  in- 
vested by  a  membrane  which  usually  contains  no  cellulose. 
They  are  commonly  abundant  in  stagnant  water  and  among 
Green  Algae  some  are  usually  present. 

Euglena  represented  in  Figure  284  is  one  of  the  most  common 
of  the  300  or  more  species  and  will  serve  to  show  the  structure 
and   habits   of  the   group.     Euglena  is   quite   commonly   seen 

swimming  about  under  the 
microscope  when  Algae  are  be- 
ing examined.  The  slender 
unicellular  body  bears  a  long 
terminal  flagellum,  has  a  chlo- 
roplast,  eye-spot,  and  pulsating 
vacuole.  These  structures  are 
characteristic  of  the  Algae, 
such  as  Volvocales  and  also  of 
protozoa,  the  one-celled  ani- 
mals. No  sexuality  is  known, 
and  multiplication  is  effected 
by  longitudinal  fission,  a 
method  characteristic  of  the 
lower  animals.  At  the  ap- 
proach of  unfavorable  condi- 
tions, as  in  autumn,  it  trans- 
forms itself  into  a  thick-walled 
resting  spore  which  germinates 
and  produces  one  or  more  new 
plants  when  favorable  condi- 
tions return.  Although  it 
usually  makes  its  own  food, 
sometimes  Euglena  loses  its 
chlorophyll  and  lives  on  organic  solutions  as  a  saprophyte,  thus 
demonstrating  that  the  saprophytic  may  readily  originate  from 
the  independent  habit. 

Many  of  the  Flagellates  change  their  forms  readily  hke  the 
Amoeba.  Sometimes  the  individuals  form  colonies  of  various 
shapes  and  often  variously  branched. 

Such  features  as  the  possession  of  chlorophyll  and  the  forma- 
tion of  thick-walled  resting  spores  suggest  a  relationship  of  the 
Flagellates  to  plants,  while  their  swimming  habits,  amoeboid 


Fig.  284.  —  A  common  species  of 
Euglena  {Euglena  gracilis).  At  the 
left,  an  adult  individual,  showing  the 
flagellum,  the  pulsating  vacuole  (p), 
the  chloroplast  (c),  and  the  nucleus 
(«)  ( X  650) ;  at  the  right  and  below, 
Euglena  in  the  spore  stage  ( X  1000) ; 
at  the  right  and  above,  a  spore  germi- 
nating and  producing  four  new  indi- 
viduals (X  1000).  Redrawn  from 
Zumstcin. 


DIATOMS 


331 


movements,  reproduction  by  longitudinal  fission,  and  such 
structures  as  contractile  vacuoles  and  red  pigment  spots  suggest 
a  relationship  to  the  animal  kingdom.  Consequently,  they  are 
regarded  as  a  transition  group  between  plants  and  animals. 

Diatoms.  —  These  one-celled  plants  are  often  classed  with  the 
Brown  Algae  on  account  of  their  brown  pigment,  although  they 
differ  from  the  Brown  Algae  in  a  number  of  ways.  The  Diatoms 
are  a  vast  assemblage  of  plants  varying  widely  in  form  and 
occurring  in  vast  numbers  in  fresh  water,  salt  water,  and  on  damp 
soil.  They  float  or  swim  commonly  on  the  surface  of  water  and 
often  in  such  vast  numbers  as  to  form  a  scum.     They  form  a  large 


Fig.  285.  —  Diatoms  of  various  kinds  (X  30-200).  In  cases  where  a  pair 
of  individuals  equal  in  length  are  shown,  two  views  of  the  same  Diatom  are 
included.     From  Kerner. 


part  of  the  floating  plankton  or  free-swimming  organic  world  on 
the  surface  of  the  ocean.  Many  occur  as  fossils  and  their  silicified 
walls  form  a  large  part  of  the  deposits  of  siliceous  earth  in  which 
form  they  are  used  in  the  manufacture  of  dynamite,  scouring 
powders,  etc.  Some  are  free-swimming  while  others  are  attached 
by  stalks. 

The  plant  body  is  microscopical  and  may  have  most  any  shape 
imaginable  as  may  be  seen  from  Figure  285.  The  cell  wall, 
consisting  largely  of  silica,  is  very  rigid  and  durable  and  is  com- 
posed of  halves  which  fit  together  one  over  the  other  much  like 
the  two  parts  of  a  pill  box.  The  walls  of  some  are  delicately 
but  beautifully  marked  with  fine  cross  lines,  which  make  certain 
Diatoms  suitable  objects  for  testing  the  definition  of  microscopes. 


332 


THALLOPHYTES 


Usually  there  is  also  a  longitudinal  line,  which  is  a  fissure  or  series 
of  fine  pores  through  which  fine  threads  of  protoplasm  project 
and  serve  like  cilia  in  locomotion.  The  halves  of  the  box-like 
shell  of  a  Diatom  are  called  valves  and  the  appearance  of  a 
Diatom  depends  much  upon  whether  the  face  of  the  valve  (the 
valve  side),  or  the  side  showing  the  joining  of  the  valves  (the 
girdle  side)  is  seen  {Fig.  286).  The  protoplast  usually  has  a  large 
central  vacuole  with  the  nucleus  suspended  in 
the  center  by  small  strands  of  cytoplasm. 

Cell  division  is  the  chief  method  of  repro- 
duction. The  cell  usually  divides  lengthwise 
and  in  such  a  way  that  the  valves  separate 
with  the  daughter  protoplasts.  Each  daughter 
protoplast  then  develops  a  new  valve  on  the 
naked  side.  In  connection  with  this,  a  pecul- 
iar situation  arises.  The  new  valve  de- 
veloped always  fits  within  the  old  one  and 
consequently  there  is  a  gradual  reduction  in 
the  size  of  the  individuals  as  division  con- 
tinues, for  at  each  division  the  daughter 
protoplast  with  the  smaller  valve  is  necessarily 
smaller  than  in  the  preceding  division.  How- 
ever, it  has  been  found  that  the  protoplasts 
shed  their  walls  when  reduction  in  size  has 
reached  a  certain  degree  and  in  a  naked 
condition  grow  to  full  size  and  then  enclose 
themselves  in  new  valves.  This  naked  pro- 
toplast is  called  an  auxospore  (meaning  en- 
larging spore). 

It  is  in  connection  wdth  these  naked  proto- 
plasts that  the  sexual  act  occurs.  Sometimes 
the  protoplasts  of  contiguous  cells  conjugate 
and  sometimes  the  four  daughter  protoplasts  of  two  contiguous 
cells  escape  and  conjugate  in  pairs.  The  zygospore  usually 
enlarges  and  then  encloses  itself  in  valves. 

Thus  Diatoms  are  one-celled  and  conjugate  like  some  Green 
Algae,  have  the  color  of  Brown  Algae  but  have  no  zoospores  or 
gametes  like  the  Brown  Algae. 

Stoneworts.  —  The  Stoneworts  constitute  the  group  scientifi- 
cally known  as  the  Charales.     Some  classify  the  Stoneworts  as 


Fig.  286.  — a 
common  Diatom, 
Navicula  viridis, 
with  valve  side 
shown  at  the  left 
and  the  girdle  side 
at  the  right.  In 
the  view  of  the 
girdle  side  one  valve 
is  seen  to  fit  over 
the  other.  From 
Strasburger. 


STONEWORTS 


333 


Green  Algae  because  they  are  green,  while  others  regard  them  as 
so  different  from  any  of  the  Algae  as  to  put  them  in  a  separate 
class.  They  grow  in  fresh  and  brackish  waters  and  often  form 
dense  masses  of  vegetation  covering  large  areas.     They  grow 


Fig.  287.  —  Chara  fragilis.  A,  part  of  a  plant,  showing  nodes,  intemodes, 
and  the  two  kinds  of  branches  (natural  size) ;  B,  part  of  a  plant,  showing  a 
node  bearing  sex  organs,  the  oogonium  enclosed  in  its  jacket  being  at  o  and 
the  antheridium  with  its  shield-shaped  wall  cells  shown  at  a  ( X  25) ;  C,  wall 
cell  of  the  antheridium,  showing  the  stalk-like  projection  at  the  end  of  which 
are  borne  the  filaments  in  the  cells  of  which  the  sperms  are  produced  (X 
about  50) ;  at  the  left  of  C,  two  cells  of  a  filament  in  which  the  sperms  are 
formed,  and  a  single  sperm  below.     Redrawn  from  Sachs  and  Thuret. 


attached  to  the  bottom  and  are  often  so  incrusted  with  calcium 
carbonate  that  they  are  rough  and  brittle  as  the  name  Stoneworts 
suggests. 

The  plant  body  has   a  much  branched  stem-like  axis  quite 
distinctly  differentiated  into  nodes  and  internodes  {Fig.  287), 


334  THALLOPHYTES 

From  the  nodes  the  branches  arise  in  whorls  and  some  branches 
resemble  leaves,  while  others  elongate  much  more  and  resemble 
the  main  axis. 

Their  reproduction  may  be  illustrated  by  following  that  of 
Chara.  There  are  no  asexual  spores,  but  the  plant  is  propagated 
vegetatively  by  special  tuber-hke  branches  which  separate  from 
the  nodes  and  grow  into  new  plants. 

Sexual  reproduction  involves  complex  structures  which  are  not 
typical  of  Algae  and  which  are  the  most  distinguishing  features 
of  the  Stoneworts.  Both  antheridia  and  oogonia  (Fig.  287)  are 
complex  structures.  Due  to  their  size  and  color  the  sex  organs 
are  visible  to  the  unaided  eye.  Both  are  developed  at  the  nodes 
and  often  close  together. 

The  antheridium  is  an  orange  or  reddish  globular  body  with 
a  wall  composed  of  eight  triangular  plate-like  cells.  From  the 
inner  side  of  each  of  the  wall  cells  there  projects  toward  the 
center  of  the  antheridium  a  much  elongated  cell  which  bears  a 
terminal  cell.  The  terminal  cell,  known  as  head  cell,  divides  into 
a  number  of  cells  and  each  of  these  produces  a  pair  of  long  fila- 
ments. Each  filament  consists  of  about  200  cells,  each  of  which 
forms  a  single  sperm.  When  an  antheridium  is  fully  formed,  its 
interior  is  a  tangle  of  filaments  and  the  sperm  output  amounts 
to  many  thousands.  The  sperm  is  a  much  elongated  ciliated 
structure,  resembling  the  sperms  of  some  of  the  more  complex 
plants  more  than  those  of  ordinary  Algae. 

The  oogonium  with  its  jacket  is  larger  and  more  elongated 
than  the  antheridium.  The  oogonia  are  often  yellow  but  are  not 
so  brightly  colored  as  the  antheridia.  An  oogonium  contains  one 
large  egg  and  much  stored  food  in  the  form  of  starch  and  oil. 
The  oogonium  is  closely  invested  by  cells  which  grow  up  from  the 
cells  below  and,  as  they  elongate,  wind  spirally  around  the 
oogonium,  forming  a  close  jacket  around  it  and  a  crown  at 
its  top. 

In  fertilization  the  cells  of  the  jacket  spread  apart  at  the 
crown,  so  that  the  sperms  can  enter.  After  fertilization  the 
jacket  hardens,  and  thus  forms  a  nut-like  case  for  the  oospore. 
When  the  oospore  germinates,  it  does  not  form  a  new  plant 
directly  but  first  forms  a  filament  of  cells,  and  the  adult  plant 
arises  as  a  branch  from  this  filament.  This  feature  is  prominent 
in  the  Bryophytes. 


STONEWORTS  335 

It  is  now  evident  that  the  Stoneworts  are  very  different  from 
the  ordinary  Green  Algae,  differing  from  them  in  structure  of 
plant  body,  character  of  sex  organs,  type  of  sperms,  and  life 
history.  They  resemble  some  of  the  more  complex  forms  of 
plants  more  than  they  resemble  the  Algae. 


CHAPTER  XIV 

THALLOPHYTES  (Continued) 

Myxomycetes  and  Bacteria  (Thallophytes  lacking  food-making 
pigments) 

There  arc  three  groups  of  Thallophytes  —  the  Myxomijcetes, 
Bacteria,  and  Fungi  —  which  are  characterized  by  the  lack  of 
food-inaking  pigments.  Having  no  chlorophyll  or  other  food- 
making  pigments,  they  are  unable  to  carry  on  photosynthesis  and 
consequently  must  depend  upon  other  organisms  for  their  food. 
Many  obtain  their  food  from  the  decaying  bodies  of  other  or- 
ganisms, while  others  attack  living  organisms. 

As  to  how  these  plants  arose,  we  are  not  certain.  Although 
some  are  the  simplest  of  plants,  they  must  have  been  preceded  by 
green  plants,  for  otherwise  there  would  have  been  no  food  for 
them.  They  are  no  doubt  degenerate  forms  of  green  plants, 
having  lost  their  food-making  pigments  as  a  result  of  their  acquir- 
ing the  habit  of  taking  food  from  other  organisms.  As  will  be 
seen  later  in  the  study  of  these  groups,  the  Bacteria  have  some  of 
the  features  of  the  Blue-green  Algae,  while  the  Fungi  present  a 
number  of  features  found  in  the  Green  or  the  higher  groups  of 
Algae.  But  for  the  Myxomycetes  we  have  no  definite  suggestions 
of  any  relationships  with  other  groups  of  plants. 

Being  dependent  plants,  these  Thallophytes  are  supposed  to 
have  evoluted  backward,  rather  than  forward.  The  Fungi,  the 
most  complex  of  the  group,  present  nothing  new  over  the  Algae 
in  the  way  of  evolution.  To  the  evolutionists  these  groups  offer 
very  little  that  is  of  interest.  They  concern  us  chiefly  because 
of  their  economic  importance, 

Myxomycetes  (Slime  Molds) 

The  plant  body  of  the  Myxomycetes,  commonly  called  Slime 
Molds,  consists  of  a  large  slimy  mass  of  protoplasm  not  enclosed 
by  cell  walls,   and  hence  the  term  myxomycetes  from  myxos 

336 


MYXOMYCETES 


337 


Fig.  288.  —  Plasiiiudium   uf  a   INIyxo- 
mycete  growing  on  wood.     X  about  |. 


(meaning  slime)  and  myces  (meaning  mold  or  fungus)  {Fig.  288). 
This  naked  mass  of  protoplasm  is  called  a  Plasmodium.  It  is  a 
semi-liquid  and  is  found  flowing  out  of  the  cracks  of  rotten  logs 
and  stumps,  forming  white  or  colored  doughy-like  masses.  They 
are  often  found  creeping  out  of  the  cracks  of  old  plank  walks,  out 
of  decayed  bark,  or  out  of 
apple  pumice  around  a  cider 
mill.  Some  of  the  Myxomy- 
cetes  are  parasites,  living  in 
the  tissues  of  higher  plants 
and  often  causing  much  in- 
jury. 

The  Plasmodium  is  multi- 
nucleate and  is  able,  by  put- 
ting out  and  withdrawing  regions  of  its  body,  to  move  about  like 
a  gigantic  Amoeba.  Sometimes  the  Plasmodium'  breaks  up  into 
many  smaller  portions  which  are  able,  by  means  of  cilia  or  flagella 
to  move  about  like  the  low  forms  of  animals.     The  Myxomycetes 

have  the  characteristics  of  both 
plants  and  animals,  and  opin- 
ions differ  as  to  whether  they 
should  be  classed  as  plants  or 
animals. 

Their  method  of  obtaining  food 
consists  chiefly  in  digesting  the 
substances      found      in     other 
plants.     Those     forms     which 
live    on    dead    organisms    are 
able    to    utilize    the    carbohy- 
drates  remaining  in    decaying 
organic    matter,    while    those 
attacking    living    plants    prey 
upon  the  tissues  of  the  plant 
attacked.    Those  forms  living  on  dead  organisms  are  called  sapro- 
phytes, while  those  forms  living  on  living  organisms  are  called 
parasites.     The  living  organism  attacked  is  called  the  host. 

Reproduction  in  the  Myxomycetes  is  asexual.  The  first  indi- 
cation of  reproduction  in  most  of  the  saprophytic  forms  is  the 
appearance  of  upward  projections  on  the  surface  of  the  Plas- 
modium.    Into    these    projections,    which    arc    at    first    hollow 


Fig.  289.—^,  Myxomycete, 
Stemonitis,  in  which  the  Plasmodium 
has  been  transformed  into  slender 
stalked  sporangia  (sp)  which  bear 
numerous  spores  (s). 


338 


THALLOPHYTES 


structures,  varying  in  shape  according  to  the  species,  the  remain- 
ing protophisni  of  the  plasmodiuni  passes  until  they  are  filled. 
Often  nearly  the  entire  plasmodiuni  is  used  in  forming  and  filling 


Fn;.  290.  —  Various  Myxomycetes,  showing  various  types  of  sporangia. 
The  large  sporangium  at  the  left  and  the  third  one  from  the  left,  below,  have 
shed  the  spores,  and  the  capillitium,  the  lace-like  framework  of  the  sporangium, 
is  i)lainly  visible.  The  larger  ones  are  larger  than  natural  size,  the  smaller 
ones  are  reduced.     From  Kerner. 

the  projections.     The  protoplasm  filling  the  upper  part  of  each 
projection  forms  numerous,  small,  globular  spores  with  heavy 


0  5  8 


Fig.  291. — Spores  of  a  My.xomycete  germinating  and  producing  motile 
animal-like  bodies  which  usually  multiply  and  later  fuse  to  form  a  Plasmo- 
dium.    Much  enlarged.     From  Woronin. 

walls,  and  thus  the  projection  becomes  a  stalked  sporangium 
{Fig.  289).  In  the  interior  of  the  sporangium  there  is  often  a 
lace-like  framework,  called  capillitium,  which  assists  through  its 
hygroscopic  movements  in  the  shedding  of  the  spores  (Fig.  290). 


SOME  MYXOMYCETES  OF  ECONOMIC   IMPORTANCE      339 


After  the  spores  are  mature,  the  wall  of  the  sporangium  breaks 
open  and  the  spores  are  scattered  far  and  near  by  wind,  animals, 
and  other  agencies.  When  the  spores  fall  on  a  suitable  object 
and  conditions  are  right,  the  protoplasm  breaks  out  of  the  heavy 
wall  and  either  grows  directly  into  a  new  Plasmodium,  or  pro- 
duces cilia,  swims  about,  and  multiplies  like  the  simple  one-celled 
forms  of  animals  {Fig.  291),  the  plasmodium  being  formed  later 
by  the  fusion  of  these  animal-like  bodies. 


Some  Myxomycetes  of  Economic  Importance 

Most  of  the  Myxomycetes  are  saprophytes  and  consequently 
the  group  is  not  so  important  economically  as  the  Bacteria  and 
Fungi.     Of  course  the  saprophytic  forms  are  of  some  importance 


Fig.  292.  —  Cabbage   plants   attacked   by   the   Club    Root    INIyxomycote 
{Plasrnodiophora  Brassicar)  which  causes  wart-like  distortions.    From  ^^■oroIlin. 


because  they  disintegrate  organic  matter  and  make  it  soluble,  so 
that  it  can  soak  into  the  soil  and  be  used  by  higher  plants. 
There  are,  however,  a  few  j^arasitic  forms  which  attack  som(>  of 
our  useful  plants  and  cause  considerable  trouble  and  loss. 


340 


THALLOPHYTES 


Club  Root  of  Cabbage.  1  —  This  is  a  disease  of  Cabbage  caused 
by  a  parasitic  Myxoinycete.  The  Myxomycete  gains  entrance 
through  the  roots  and  lives  upon  the  cells  of  the  plant.  The 
presence  of  the  parasite  causes  the  wart-like  developments  on  the 
roots  and  stem  of  the  Cabbage,  and  so  injures  the  plant  that 
no  head  is  produced  and  even  death  often  results  {Figure  292). 

Within  the  cells  of  the 
Cabbage  the  plasmodia  live 
and  form  spores  {Figure 
293).  When  liberated 
through  the  decay  of  the 
Cabbage,  the  spores  are 
carried  by  water,  animals, 
or  wind  to  other  plants. 
The  spores  may  lie  in  the 
ground  and  infect  plants  in 
succeeding  years.  This  dis- 
ease is  not  only  destructive 
to  Cabbage  but  often  at- 
tacks Turnips,  Radishes, 
Rutabagas,  and  Cauli- 
flower. The  important  fea- 
ture in  controlling  the 
disease  consists  in  prevent- 
by  burning  infected  plants, 
sulphur,     and    rotation    of 


"^"^^^4^-'^'* 


Fig.  293.  —  Cross  section  of  a  root 
of  Cabbage  affected  with  Club  Root, 
showing  the  plasmodia  (p)  within  the 
tissues.     From  Woronin. 


ing   the   spores   from   functioning 
treating    the    soil    with    lime    or 
crops. 

Powdery  scab  of  the  Irish  Potato.^  —  This  disease  is  caused  by 
one  or  more  kinds  of  Myxomycetes  which  enter  the  tubers  and 
roots  of  the  Irish  Potato  and  destroy  the  tissues  {Fig.  29^).  The 
Amoeba-like  plasmodia  live  in  the  cells,  which,  due  to  the  presence 

1  Cabbage  Club  Root  in  Virginia.  Bulletin  191,  Virginia  Agr.  Exp.  Sta., 
1911. 

Studies  on  Club  Root.     Bulletin  175,  Vermont  Agr.  Exp.  Sta.,  1913. 

Studies  on  Clubroot  of  Cruciferous  Plants.  Bulletin  387,  Cornell  Uni- 
versity Agr.  Exix  Sta.,  1917. 

-  Powdery  Scab  (Spongospora  subterranea)  of  Potatoes.  Bulletin  S2, 
U.  S.  Dept.  Agr.,  1914. 

Powdery  Scab  of  Potatoes.     Bulletin  227,  Maine  Agr.  Exp.  Sta.,  1914. 

Spongospora  subterranea  and  Phoma  tuborosa  on  the  Irish  Potato,  Vol.  7, 
No.  5,  pp.  213-254,  Jour.  Agr.  Research,  U.  S.  Dept.  Agr.,  1916. 


BACTERIA 


341 


of  the  organism,  develop  abnormally,  producing  scabby  formations 
which  constitute  the  scabby  areas  on  the  tuber  or  root.  The 
Plasmodia  are  finally  transformed  into  spores  which  are  liberated 
as  powdery  masses  as  the  infected  tissues  die  and  the  spore  masses 
break  open.  It  has  been 
found  that  the  spores  can 
live  in  the  ground  for  a  num- 
ber of  years  and  may  also 
live  adhering  to  the  rind  of 
the  Potato.  Treating  seed 
Potatoes  with  weak  solu- 
tions of  formaldehyde  or 
corrosive  sublimate  to  kill 
the  spores  adhering  to  the 
tubers,  and  rotating  crops, 
so  that  the  Potatoes  are 
not  planted  in  infected  soil 

are  means  of  controlling  the      Fig.  294.  —An  lush  Potato  attacked  by 
disease.  a  Myxomycete,  Spongospora  svbterranea. 

Bacteria 


Bacteria,  of  which  there  are  1400  or  more  species,  are  the 
smallest  of  plants,  and  their  study  requires  microscopes  of  a  very 
high  power  of  magnification.  Some  spherical  forms,  visible  only 
through  the  best  microscopes,  are  less  than  0.0005  of  a  millimeter 
in  diameter,  and  some  Bacteria  are  known  to  exist  that  are 
ultramicroscopic,  that  is,  too  small  to  be  seen  with  the  best 
microscopes.  They  are  present  almost  everywhere,  occurring 
in  the  soil,  in  water,  in  the  air,  and  in  all  organic  bodies  living  or 
dead.  Although  so  insignificant  in  size,  they  are  of  great  im- 
portance, because  the  service  of  some  forms  is  indispensable  to 
our  welfare,  while  the  forms  which  cause  diseases  are  destructive 
to  both  plants  and  animals.  The  disease-producing  forms  are 
commonly  known  as  germs  or  microbes.  So  numerous  and 
important  are  these  simple  plants  that  their  study  now  forms  a 
special  subject  called  Bacteriologj/. 

The  plant  body  of  the  Bacteria  consists  of  a  single  cell.  Bac- 
teria are  of  three  general  forms:  coccus  forms,  which  are  globular; 
bacillus  forms,  in  which  the  shape  is  rod-like;  and  spirillum 
forms,   in  which   the   plant   body  is  a  curved  rod   (Fig.  295). 


342 


THALLOPHYTES 


Many  of  the  Bacteria  are  provided  with  ciha  or  terminal  flagella, 
Avhich  enable  them  to  move  about  independently.  The  cilia  are 
distributed  over  the  body  in  various  ways  and  are  extremely 
difficult  to  detect.     Some  of  the  motile  forms  are  quite  active 

and  motility  is  one  of  the  fea- 
tures suggesting  that  Bacteria 
are  animals.  Their  cell  walls 
are  more  or  less  slimy,  and  their 
protoplasm  is  not  definitely  or- 
ganized into  nucleus  and  cyto- 
plasm. These  features  with 
their  power  of  resistance  suggest 
a  relationship  with  the  Blue- 
green  Algae.  They  possess  no 
chlorophyll  and  are  almost  ex- 
clusively parasites  or  sapro- 
phytes. The  ability  of  the 
protoplasm  to  endure  extreme 
cold,  high  temperatures,  and 
drying  even  surpasses  that  of 
the  Blue-green  Algae.  Besides 
remaining  separate  or  forming 
filaments,  Bacteria  commonly  have  another  stage  in  which  numer- 
ous individuals  are  held  together  in  masses  or  colonies  by  a  matrix 
of  gelatinous  substance  formed  from  their  walls.  This  stage 
is  known  as  the  zoogloea  stage  {Fig.  296).  These  colonies  form 
the  characteristic  pellicles  on  nutrient  media,  as  on  the  water  in 
which  hay.  Beans,  Peas,  or  other  organic  substances  are  decay- 
ing, and  on  bouillon  and  various  solid  media  {Fig.  297).  When 
food  is  scarce  or  other  conditions  unfavorable,  some  forms  shrink 
their  protoplasm  and  enclose  it  in  an  inner  heavy  wall,  thus  form- 
ing what  is  called  a  spore.  Enclosed  in  this  heavy  wall,  they  are 
inactive  and  extremely  resistant  to  cold,  heat,  and  drying.  When 
transferred  by  wind  or  other  agents  to  a  suitable  medium,  they 
shed  the  heavy  wall  and  become  active  again. 

Their  method  of  getting  food  is  essentially  the  same  as  in  the 
Myxomycetes.  Since  they  live  on  or  Avithin  the  food  supply, 
they  are  in  direct  contact  with  the  food  material,  and  have  only 
to  change  it  to  a  soluble  form  and  absorb  it  through  their  walls. 
They  secrete  enzymes  which  change  insoluble  foods  to  soluble 


Fig.  295.  —  Some  forms  of  Bac- 
teria. At  the  right  and  above,  a 
coccus  form;  a  bacillus  just  below; 
and  a  spirillum  form  at  the  bottom. 
At  the  left,  above,  a  chain  of  bacilU; 
and,  below,  bacteria  in  the  spore 
Very  highly  magnified. 


BACTERIA 


343 


forms  and  as  a  result  of  their  activity  various  substances  are 
produced,  the  accumuhition  of  which  check  their  activity. 
Some  forms,  called  anaerobic,  get  along  better  without  air,  while 
others,  called  aerobic,  nuist  have  air. 

Their  reproduction  is  accomplished  l)y  cell  division,  which  is 
not  so  complex  and  takes  place  more  rapidly  than  in  the  cells  of 


wmm 


''lillr 

Fig.  296.  —  Bacillus  sublilis,  a  Bacterium  of  decay.  Above,  the  active 
form  (X  1500);  at  the  left,  below,  spore  stage  (X  800);  at  the  right,  below, 
the  zoogloea  stage  ( X  500) . 


the  higher  plants.  Cell  division  is  so  rapid  that  the  progeny  of 
one  individual  often  runs  into  many  millions  in  twenty-four 
hours.  The  new  individuals  may  separate  immediately  after 
division  is  complete  or  cling  together  in  filaments.  Sometimes 
in  shrinking  the  protoplasm  and  enclosing  it  in  an  inner  heavy 
wall  in  preparation  for  the  resting  stage,  the  protoplasm  divides 
and  each  separate  mass  of  protoplasm  forms  a  spore.  Since  each 
spore  is  an  individual  in  a  dormant  protected  state,  the  formation 
of  more  than  one  spore  results  in  the  multiplication  of  individuals. 


344  THALLOPHYTES 

But  u  Bacterium  commonly  forms  only  one  spore,  and  the 
formation  of  spores,  therefore,  does  not  generally  result  in  the 
multiplication  of  individuals.  The  spore  stage  is  apparently  for 
protection  rather  than  for  multiplication.  In  the  spore  stage 
Bacteria  can  live  where  there  is  no  food  and  when  heat  and 


Fig.  297.  —  A  petri  dish  containing  agar  upon  which  are  colonies  of  the 
Bacterium  (Actinomyces  chromogenus)  which  attacks  Irish  Potatoes,  causing 
scabby  areas.  The  white  spots  are  the  colonies.  From  Bulletin  184,  Ver- 
mont Agr.  Exp.  Sta. 

cold  are  much  too  extreme  for  ordinary  life.  In  this  condition 
some  Bacteria  can  endure  boiling  water  for  an  hour  or  longer  as 
well  as  extremely  low  temperatures.  It  is  the  spores  of  Bacteria 
that  are  hard  to  kill  in  sterilizing  media  and  other  substances. 
In  the  spore  stage  Bacteria  can  retain  their  vitality  for  months 
or  years  and,  floating  about  with  the  dust  in  the  air,  reach  all 
kinds  of  situations. 

Some  Bacteria  of  Economic  Importance 

Bacteria  of  Decay.  —  The  Bacteria  of  decay  hve  on  dead 
organisms.  By  their  activity  dead  organisms,  such  as  straw, 
weeds,  corn  stalks,  logs,  and  carcasses  of  animals,  are  decomposed 
into  simpler  and  soluble  compounds,  in  which  form  they  return 
to  the  earth  and  become  available  nutrients  for  plants.  Their 
activity  helps  to  rid  the  earth's  surface  of  debris  and  to  pre- 


BACTERIA  OF  NITRIFICATION  AND  NITROGEN  FIXATION     345 

vent  the  soil  from  becoming  depleted  of  plant  nutrients.  Of 
course  they  attack  meats,  canned  fruits,  and  many  other  things 
which  we  do  not  wish  to  have  decomposed,  but  the  good  they  do 
more  than  compensates  the  harm.  Methods,  such  as  cold  stor- 
age, applications  of  salt  and  other  preservatives,  and  canning,  are 
employed  in  checking  or  preventing  the  activity  of  Bacteria  in 
foods.  In  cold  storage  the  temperature  is  too  low  for  them  to  be 
active.  Salt  solutions  keep  them  dormant  by  extracting  water 
from  them.  In  canning  those  present  are  killed  by  heat,  and  by 
seaHng  the  cans  others  are  prevented  from  entering.  Alcohol, 
formaldehyde,  car])olic  acid,  etc.,  are  useful  in  preventing  bac- 
terial action  in  materials  not  intended  for  food. 

Bacteria  of  Fermentation.  —  These  Bacteria  attack  carbohy- 
drates and  break  them  into  simpler  substances,  such  as  alcohol, 
lactic  acid,  acetic  acid,  butyric 

acid,    etc.     A    few    forms    are  c:^^^^         '^'^cscs 

shown  in  Figure  298.  The  _a^^^^^  c^'?^^^'^ 
product  produced  depends 
upon  the  substance  attacked 
and  the  kind  of  Bacteria  at 
work.  For  example  in  the 
fermenting  of  cider,  some  Bac- 
teria    break     the     sugar     into        .  ^ig.  298.  —  Bacteria  of  fermenta- 

alcohol    and    carbon    dioxide,      j°"„    "'  .' ^^\    c,  vinegar     ac  eria, 

'       d.    Bacteria   that   ferment   milk;     e, 
while   others   attack   the   alco-      butyric  acid  Bacteria.    X  1000.    Re- 
hol,    changing    it    into    acetic      drawn  from  Fisher. 
acid.      All  the  forms  working 

together  change  the  cider  into  vinegar.  After  the  vinegar  Bac- 
teria become  inactive,  due  to  the  exhaustion  of  the  food  supply 
or  the  accumulation  of  the  fermented  products,  they  form  the 
well-known  mother  of  vinegar,  which  consists  of  the  Bacteria 
held  together  in  a  gelatinous  matrix.  In  milk,  certain  kinds  of 
Bacteria  attack  the  milk  sugar  and  change  it  into  lactic  acid. 
Another  kind  produces  butyric  acid  in  butter,  turning  it  rancid. 
Bacteria  of  Nitrification  and  Nitrogen  Fixation.  —  In  the  soil 
there  are  some  kinds  of  Bacteria  that  change  certain  nitrog- 
enous compounds  of  manure  and  other  organic  matter  into 
nitrates  in  which  form  the  nitrogen  is  available  for  crops.  Tlie 
advantage  to  the  Bacteria  is  that  they  secure  energy  in  tliis 
way  from  these  compounds,  while  the  advantage  to  the  soil  is 


346 


THALLOI'HV^rES 


.> 


that  tlie  nitrost'ii  of  the  coinpouiids  decomposed  is  put  in  an 
avaihU)le  form  for  other  phmts.  The  Bacteria  use  the  chemical 
energy  dciixcd  from  the  oxidation  of  organic  compounds  in  per- 
forming the  work  involved  in  building  up  their  bodies.  There 
are  a  few  exceptional  forms  of  soil  Bacteria  which  can  actually 

make  their  own  food,  and 
this  they  do  by  using  this 
chemical  energy,   as  green 
^^-^nn^M^         plants  do  sunlight,  in  the 
hIk  ^^^k^^^i  construction  of  foods  from 

^^E^^^^^^HBB^^I  carbon  dioxide  and  water. 

'  ^^^H^I^^^H^^^^W  There         other  kinds 

:  ■' ^^^^^^^^^^^^^^  I  soil   Bacteria   which   have 

the  power  of  actually  in- 
creasing the  nitrogen  in  the 
soil.  They  incorporate  the 
gaseous  nitrogen  of  the  air 
into  nitrogen  compounds, 
which  they  use  in  building 
up  their  own  bodies,  and 
when  their  bodies  decay, 
these  nitrogenous  com- 
pounds are  added  to  the 
soil,  which  is  therel^y  en- 
riched. Some  kinds  of 
these  Bacteria  live  inde- 
pendently in  the  soil,  while 
some  kinds  are  associated 
with  higher  plants,  espe- 
cially the  Legumes,  such  as 
Clover,  Alfalfa,  Beans,  etc. 
{Fig.  299) .  They  enter  the 
roots  of  these  plants,  and, 
as  a  result  of  the  attack, 
the  roots  form  nodules  in 
which  the  Bacteria  live  and  carry  on  their  work  of  fixing  nitro- 
gen. It  is  due  to  their  association  with  these  Bacteria  that  the 
Legumes  are  im])ortant  in  enriching  the  soil. 

Pathogenic  bacteria.  —  These  are  the  disease-producing  forms. 
They  prey  upon  both  animals  and  plants.     The  disease  is  the 


Fig.  299.  —  A  young  Red  Clover  plant, 
showing  the  root  nodules  that  are  associated 
with  the  nitrogen  fixing  Bacteria.  From 
Farmer's  BuUelin  435,  U.  S.  Dept.  of  Agri- 
culture. 


BLACK-IlOl^  OF   CJABBACE 


347 


C^o'^c 


result  of  their  direct  attack  upon  the  tissues,  or  of  the  poisons, 
called  toxins,  which  they  excrete.  They  produce  most  of  the 
diseases  of  human  beings,  such  as  erysipelas,  tetanus,  diphtheria, 
tuberculosis,  typhoid  fever,  pneumonia, 
cholera,  etc.  (Fig.  300).  Among  our 
domesticated  animals,  such  diseases  as 
hog  cholera,  splenic  fever,  glanders, 
black-leg,  etc.,  are  caused  by  Bacteria. 
The  fighting  of  these  forms,  either  to 
exclude,  destroy,  or  neutralize  them,  is 
the  business  of  modern  medicine  and 
surgery.  Besides  the  dangerous  forms 
which  attack  animals,  there  are  numer- 
ous harmless  forms  constantly  present 
throughout  the  alimentary  canal. 

Among  plants   the   disease-producing 
Bacteria  are  almost  as  busy  as  among 

animals.     Not  only  the  tender  herbaceous  plants  but  even  the 
trees  are  attacked,  and  the  loss  caused  every  year  is  large. 


Fig.  300.— Some  patho- 
genic Bacteria.  o,  pus 
cocci;  h,  erysipelas  cocci; 
c,  Bacteria  causing  diph- 
theria; d,  typhoid  bacilh. 
X  1500.  Redrawn  from 
Fisher. 


Fig.  301.  —  Potato  tuber  affected  with  the  Potato  Scab  caused  by  a 
Bacterium,  Actinomyces  chromogemis .  From  Bulletin  184,  Vermont  Agr. 
Exp.  St  a. 


Black-rot  of  Cabbage.  —  This  disease  occurs  on  Cabbage, 
Turnips,  and  other  plants  of  this  family.  The  Bacteria  enter 
through  the  openings  of  the  leaf  and  advance  through  the 
vascular  bundles.     They  are  able  to  destroy  cell  walls  as  well 


348 


THALLOPHYTES 


as  the  living  content  of  the  cells.  Their  presence  in  the  leaf 
causes  a  blackening  of  the  veins  and  a  yellowing  of  the  mesophyll. 
The  disease  may  spread  to  the  stem,  where  it  clogs  the  vascular 
bundles  and  destroys  tissues.  Plants  attacked  lose  their  leaves 
and  are  dwarfed  or  killed. 

Potato  Scab.^  —  There  are  a  number  of  organisms  which  at- 
tack the  Irish  Potato  and  cause  scabby  areas  and  the  decay  of 

the  tuber.  Among  this 
group  of  organisms  pro- 
ducing scab  there  is  one 
of  the  higher  forms  of 
Bacteria  scientifically 
called  Actinomyces  chromo- 
genus  (Fig.  301).  Among 
other  bacterial  diseases  of 
the  Irish  Potato,  Black- 
leg^ is  of  considerable  im- 
portance, especially  in  the 
Southern  States. 

Pear  Blight.^  —  This  dis- 
ease occurs  on  many  fruit 
trees,  but  is  more  serious 
on  Pears  and  Apples.  It 
is  often  called  Fire  Blight 
or  Blossom  Blight.  The 
Bacteria  enter  the  young 
twigs,  usually  through  the 
flowers,  and  attack  the 
cambium  and  cortex.  The 
tips  of  the  twigs  with  their 
flowers  and  leaves  soon 
wilt,  and  in  a  few  weeks 
blacken  and  die.  Sometimes  when  the  attack  is  quite  general, 
scarcely  a  flower  tip  of  an  infected  tree  escapes.  This  not 
only  results  in  loss  of  fruit,  but  the  tree  is  often  so  disabled 
that   death  results.     Figure  302  shows    a   Pear   twig    severely 


Fig.  302.  —  Fire  blight  on  the  Pear. 
The  tip  of  the  branch  is  being  killed  by 
the  Bacteria.     After  Whetzel  &  Stewart. 


1  Potato  Scab.     Bulletin  184,  Vermont  Agr.  Exp.  Sta.,  1914. 

2  Potato  Tuber  Diseases.     Farmer's  Bulletin  5U,  U.  S.  Dept.  Agr.,  1913. 

^  Fire  Blight  Disease  in  Nursery  Stock.     Bulletin  329,  Cornell  University 
Agr.  Exp.  Sta.,  1913. 


CROWN  GALL 


349 


attacked.  The  Bacteria  pass  the  winter  in  the  infected  regions, 
which  are  sources  of  further  infection.  When  growth  begins  in 
the  spring,  a  gummy  substance  carrying  the  Bacteria  exudes 
from  these  dead  portions,  and  insects  visiting  the  exudations 
carry  the  disease  to  other  trees.  How  would  you  combat  Pear 
Bhght? 

Crown  Gall.  —  This  disease  is  common  on  fruit  trees,  and 
occurs  on  Roses,  Blackberries,  Alfalfa,  and  a  number  of  other 
plants.  The  presence  of  the  Bac- 
teria causes  an  abnormal  develop- 
ment of  the  infected  tissues,  resulting 
in  the  formation  of  cancer-like  swell- 
ings. The  disease  may  occur  on 
any  portion  of  the  plant  but  is  com- 
mon on  the  roots  or  on  the  stem 
near  the  surface  of  the  ground.  In 
general,  nursery  stock  is  more  readily 
affected  than  older  trees.  Plants 
affected  are  much  dwarfed  or  killed 
(Fig.  303). 

Beans,  Tomatoes,  Potatoes,  Sugar 
Cane,  Cotton,  and  most  of  our  eco- 
nomic plants  have  some  form  of 
bacterial  disease,  but  a  further  study 
of  the  disease-producing  forms  must 
be  left  to  courses  in  Bacteriology 
and  Pathology. 

In  summarizing,  the  following  fea- 
tures should  be  noted.  Bacteria  are 
the  smallest  of  plants,  and  their 
plant  body  consists  of  a  single  cell 
with  protoplast  poorly  organized. 
The  plant  body  may  be  globular 
or  rod-shaped,  either  straight  or  curved.  Some  have  cilia  or 
flagella  and  are  therefore  motile.  They  are  remarkably  re- 
sistant, especially  in  the  spore  stage.  With  the  exception  of  a 
few  forms,  they  are  saprophytes  or  parasites.  The  disease- 
producing  forms  are  very  destructive  to  both  animals  and  plants. 
They  reproduce  by  rapid  cell  division  and  are  spread,  partly  by 
wind,  partly  by  water  currents,  partly  by  their  own  locomotion, 


Fig.  303.  —  Crown  Gall  on 
the  Cherry  tree.  The  cancer- 
like swellings  are  due  to  the 
presence  of  Bacteria.  After 
Bulletin  235,  California  Agr. 
Exp.  Sta. 


350  THALLOPHYTES 

and  partly  by  the  movements  of  animals  with  which  they  are 
associated.  Their  structural  simplicity,  power  of  resistance,  and 
gelatinization  of  walls  suggest  a  relationship  to  the  Blue-green 
Algae. 

In  connection  with  Bacteria  another  group  of  organisms,  known 
as  the  Myxohacteria,  is  commonly  discussed.  As  the  name  sug- 
gests, the  mj'xobacteria  resemble  both  the  Bacteria  and  Myxo- 
cytes.  They  differ  from  the  Bacteria  in  that  they  form  colonies, 
which  in  some  cases  are  definite  and  elaborate  in  form.  Some 
form  colonies  resembhng  a  stalk  bearing  a  group  of  sporangia  at 
its  summit.  The  life  histories  of  the  individuals  of  a  colony  are 
essentially  the  same  as  those  of  the  Bacteria,  differing  chiefly  in 
that  the  resting  cells  or  spores  form  rod-like  cells  which  escape 
and  assemble  .to  organize  the  colony. 


CHAPTER   XV 

THALLOPHYTES    (Concluded) 

Fungi  (Thallophytes  Lacking  Food-making  Pigments) 

General  Discussion.  —  The  Fungi  are  a  very  large  group  of 
Thallophytes.  There  are  thousands  of  different  kinds  of  Fungi. 
Most  people  know  some  of  the  common  forms,  such  as  Toadstools, 
Mushrooms,  and  Puffballs,  and  those  who  live  on  the  farm  are 
probably  acquainted  with  the  Rusts  and  Smuts  of  our  common 
cereals.  Most  of  the  plant  diseases  are  caused  by  Fungi.  Like 
the  Slime  Molds  and  Bacteria,  they  have  no  food-making 
pigment  and  consequently  are  either  saprophytes  or  parasites. 
They  attack  both  animals  and  plants.  Plant  Pathology,  which 
is  a  study  of  plant  diseases,  devotes  some  time  to  the  study  of 
Slime  Molds  and  Bacteria,  but  is  concerned  mainly  with  the 
Fungi.  They  attack  vegetables,  grains,  fiber  plants,  fruits,  fruit 
trees,  and  forest  and  shade  trees.  The  destruction  which  they 
cause  is  enormous.  Some  of  the  parasitic  forms,  however,  are 
harmless,  and  many  of  the  saprophytic  forms  are  beneficial. 

It  is  generally  supposed  that  the  Fungi  are  derived  from  the 
Algae,  having  lost  their  chlorophyll  and  independent  living. 
Some  of  them  have  plant  bodies,  zoospores,  sex  organs,  and  sex 
cells  similar  to  those  of  the  Green  Algae,  while  some  have  sex 
organs  resembling  those  of  the  Red  Algae,  but  have  no  resemblance 
in  other  features.  Some  have  become  so  modified  by  their  de- 
pendent habit  of  living  that  they  have  lost  all  of  their  alga-like 
features.  They  have  made  no  advancement  in  evolution,  for 
there  is  less  differentiation  of  plant  body  in  this  group  than 
in  the  Algae,  and  methods  of  reproduction  show  no  improvement, 
but  often  are  simpler  than  those  of  the  Algae  or  have  been  lost 
entirely.  Those  botanists  who  study  plants  mainly  from  the 
standpoint  of  evolution  devote  very  little  time  to  the  Fungi 
because  they  have  contributed  nothing  to  evolution.  But  from 
the  economic  standpoint,  the  Fungi  are  an  exceedingly  important 

351 


352  THALLOPHYTES 

group.  A  knowledge  of  their  plant  bodies,  methods  of  reproduc- 
tion, and  how  they  injure  other  plants  is  essential  for  working 
out  methods  of  controUing  the  destructive  forms. 

The  plant  body  of  Fungi  consists  of  a  mass  of  colorless  branch- 
ing threads  or  filaments,  and  is  called  a  mycelium  (plural  mycelia). 
The  individual  threads  are  called  hyphae  (singular /lyp/ia) .  The 
hyphae  constituting  a  mycelium  may  be  loosely  interwoven, 
forming  a  structure  resembling  a  delicate  cobweb,  as  in  the  Bread 
Mold,  or  they  may  be  woven  into  a  compact  body  having  a 
definite  shape,  such  as  occurs  in  Toadstools  and  Mushrooms. 
The  mycelium  must  be  in  direct  contact  with  its  food  supply, 
which  is  called  the  substratum. 

Sometimes,  especially  in  the  case  of  parasites,  special  short 
branches  are  formed  which  penetrate  the  host  and  absorb  food 
material.  These  special  absorbing  branches  are  called  haustoria, 
meaning  "absorbers." 

Hyphae  are  modified  in  various  ways  for  reproduction.  Some 
produce  spores,  which  are  sometimes  borne  in  sporangia  and 
sometimes  openly  on  the  end  or  sides  of  the  hyphae.  Some  are 
modified  into  organs  for  bearing  sex  cells.  These  various  modi- 
fications for  reproduction  will  be  learned  as  the  different  groups 
and  their  types  are  studied. 

Divisions  of  Fungi.  —  The  Fungi  are  so  much  modified  by 
their  peculiar  life  habits  that  they  have  either  lost  or  disguised 
the  structures  which  prove  most  helpful  in  the  classification  of  the 
Algae.  The  Fungi  are  divided  into  four  large  subdivisions,  but 
the  life  histories  of  only  three  of  the  subdivisions  are  well  known. 

The  constant  termination  of  the  group  names  is  mycetes,  a 
Greek  word  meaning  "Fungi."  To  this  name  is  added  a  prefix 
which  is  intended  to  indicate  some  important  character  of  the 
group.  The  three  subdivisions  in  which  the  life  histories  are 
known  are  named  as  follows:  (1)  Phycomycetes  (Alga-like 
Fungi)  "phyco"  coming  from  "phykos,"  meaning  Seaweed  and 
suggesting  the  water  habits  of  this  group;  (2)  Ascomycetes  (Sac 
Fungi),  so  named  because  they  bear  spores  in  small  sacs  called 
asci  (singular  ascus);  and  (3)  Basidiomycetes,  Fungi  which  bear 
spores  on  small  club-shaped  hyphae  called  basidia  (singular  basid- 
ium).  To  the  Basidiomycetes  belong  such  familiar  forms  as 
the  Toadstools,  Mushrooms,  Rusts,  and  Smuts.  The  fourth 
group  is  known  as  the  Fungi  Imperfecti.     They  are  those  Fungi 


WATER   MOLD   (SAPROLEGNIA) 


353 


which  are  not  known  to  have  a  Ufe  history  of  the  type  character- 
istic of  either  of  the  other  subdivisions.  Their  hfe  histories  so 
far  as  they  are  known  are  imperfect. 


Phycomycetes  (Alga-like  Fungi) 

This  group,  as  their  name  suggests,  resembles  the  Algae,  but  it 
is  a  large  assemblage  of  plants  which  vary  widely  in  both  struc- 
ture and  habit.  Some  of  them  live 
in  the  water  and  have  zoospores 
and  sex  organs  similar  to  those  of 
the  Green  Algae,  while  some  have 
lost  their  water  habits  and  nearly 
all  of  their  algal  features.  There 
are  three  principal  orders  —  Water 
Molds  (Saprolegniales),  Downy 
Mildews  (Peronosporales) ,  and 
True  Molds  (Mucorales). 

Water  Molds  (Saprolegniales). — 
These  are  the  Fungi  showing  closest 
affinities  with  the  Algae.  In  fact, 
if  some  of  them  had  chlorophyll, 
they  could  scarcely  be  told  from  ^^  304  _  ^  ^r^^^.  Mold 
some  of  the  Green  Algae.  They  (Saprolegnia) .  ^,  a  fly  affected 
live  in  the  water  where  they  feed  with  Saprolegnia.  The  fuzzy  ap- 
upon  the  dead  bodies  of  insects,  pearance  of  the  fly  is  due  to  nu- 
fish,  and  tadpoles.  Sometimes  they  ^b^dy^of  ^h:^'"?^^^^^^^^^ 
attack  hvmg  organisms,  as  the  one  projecting  hyphae  which 
called  Saprolegnia  illustrates. 

Water  Mold  (Saprolegnia).  — 
There  are  several  kinds  of  Water 
Molds,  but  the  one  called  Sapro- 
legnia, shown  in  Figure  304,  is  a 
very  common  one.  Although  com- 
monly a  saprophyte  on  the  floating  pierced  the  wall  of  the  oogonium 
bodies  of  dead  organisms,  it  often  and  thereby  enabled  the  sperms 
attacks  and  kills  young  fish  that  to  reach  the  eggs. 
are  confined  in  close  quarters,  on  which  account  it  is  some- 
times very  destructive  in  fish  hatcheries.  This  Fungus,  since 
it  can  live  on  both  live  and  dead  organisms,  shows  that  there 


have 

formed  zoosporangia.  From  the 
zoosporangium  at  the  right  the 
zoospores •  are  escaping.  C,  a 
tip  of  a  projecting  hypha  bearing 
an  oogonium  containing  a  number 
of  eggs.  D,  an  oogonium  and  an- 
theridium,  the  latter  of  which  has 


354  THALLOPIIYTES 

is  no  sharp  line  of  distinction  between  parasites  and  saprophytes. 
As  long  as  the  host  is  living,  the  Fungus  is  a  parasite,  but  upon 
the  death  of  the  host  it  becomes  a  saprophyte.  Thus  a  Fungus 
may  be  a  parasite  at  one  time  in  its  life  and  a  saprophyte  at 
another.  Saprolegnia  is  usually  obtained  for  study  by  throwing 
dead  insects  or  pieces  of  beefsteak  into  stagnant  water  from  a 
pond,  where  the  objects  usually  become  infected  and  soon  look 
like  the  fly  in  Figure  304. 

The  mycelium  of  Saprolegnia  is  composed  of  many  branched 
hyphae  which  extend  throughout  the  tissues  of  the  host.  The 
hyphae  are  coenocytes.  This  coenocytic  feature  suggests  a  closer 
kinship  to  Vaucheria  than  to  the  other  Green  Algae.  After  the 
mycelium  is  well  established  in  the  host,  numerous  hyphae,  which 
cause  the  fuzzy  appearance,  protrude  from  the  surface  of  the. 
host. 

The  hyphae  within  the  tissues  of  the  host  are  able  to  absorb 
food  materials  directly.  They  are  also  able  by  means  of  enzymes 
to  change  materials  to  soluble  forms,  and  in  this  way  the  Water 
Molds  bring  about  the  decay  of  animal  bodies  in  water. 

Many  of  the  hyphae  protruding  from  the  host  become  modified 
for  reproduction.  Some  produce  zoospores,  while  others  produce 
sex  organs.  The  swollen  tips  of  some  of  the  protruding  hyphae 
are  cut  off  by  a  cross  wall  and  form  sporangia  in  which  are  pro- 
duced numerous  zoospores.  These  zoospores  escape,  swim  about, 
and  when  in  contact  with  another  host  produce  hyphae  that 
penetrate  and  infect  the  new  host. 

Oogonia  and  antheridia  are  also  formed  at  the  ends  of  hyphae. 
The  oogonia  are  spherical  and  form  one  and  sometimes  many 
eggs.  The  antheridia  are  formed  on  branches  near  the  oogonia. 
The  antheridium  comes  in  contact  with  the  oogonium  and  pierces 
its  wall  with  a  small  tube  through  which  the  sperms  from  the 
antheridium  pass  and  fertilize  the  eggs.  As  a  result  of  fertiliza- 
tion, a  heavy-walled  oospore  is  formed,  which  after  rest  grows 
into  a  hypha  which  can  penetrate  and  infect  a  host. 

A  peculiar  feature  in  connection  with  some  of  the  Saprolegnias 
is  the  ability  of  their  eggs  to  develop  without  fertilization.  In 
most  plants,  unless  the  egg  is  fertilized,  it  will  not  develop,  but 
will  soon  disintegrate  and  disappear.  In  some  Saprolegnias,  the 
sperms  of  the  antheridium  fail  to  enter  the  oogonium,  or  there 
may  be  no  antheridium  developed,   and  still  the  egg  without 


THE  GRAPE  DOWNY   MILDEW   (PLASMOPARA   VITICOLA)    355 

fertilization  fonns  an  otispore  whi(;h  can  gcnninato.  This 
peculiar  feature,  called  parthenogenesis,  meaning  reproduction 
by  an  egg  without  fertilization,  has  been  mentioned  befoi-e 
(page  50). 

The  aquatic  habit,  reproduction  by  zoospores,  and  character 
of  sex  organs  support  the  theory  that  the  Water  Molds  are 
degenerate  forms  with  the  Green  Algae  as  their  ancestors.     The 


Fig.  305.  —  Leaf  of  the  Grape,  showing  the  downy  areas  caused  by  the 
Downy  Mildew,  Plasmopara  Viticola.     After  W.  H.  Hein. 

coenocytic  character  of  their  hyphae  suggests  a  close  relationship 
with  the  Siphonales. 

Downy  Mildews  (Peronosporales).  —  These  Fungi,  which  are 
parasites  on  the  higher  plants,  cause  some  serious  plant  diseases 
of  which  the  Grape  Downy  Mildew  and  Potato  Blight  are  notable 
ones,  and  will  serve  to  illustrate  the  habits  of  the  group.  There 
are  about  100  species  known  and  the  order  is  so  named  because 
of  the  downy  patches  which  they  produce  on  the  diseased  portions 
of  the  host. 

The  Grape  Downy  Mildew  (Plasmopara  Viticola).  —  The 
Downy  Mildew  of  the  Grape,  shown  in  Figure  305,  is  a  very 


356 


THALLOPHYTES 


coinnion  and  important  one  of  the  many  Downy  Mildews  and 
often  causes  much  loss  in  grape-growing  districts.  Its  downy 
white  growth  occurs  most  commonly  on  the  leaves,  but  the  Fun- 
gus often  attacks  the  green  shoots  and  fruit  {Fig.  306).  Some- 
times it  destroys  the  fruit  crop  and  weakens  the  vines. 

The  mycelium  consists  of  coenocytic  hyphae,  which  extend 
through   the   tissues  of  the  part  at- 
tacked.     The  hyphae  grow  between 
>'£!^  4Ps-  cells    and   send   into   the   cells   short 

branches  (haustoria)  which  absorb 
the  cell  contents  of  the  host  {Fig. 
307).  The  death  of  the  leaf  cells  re- 
sulting from  the  attack  is  indicated 
by  the  occurrence  of  yellow  or  brown 
areas  which  may  involve  much  of 
the  leaf.  This  destruction  of  leaf 
tissue    diminishes   the    carbohvdrates 


Fig.  306. —  A  bunch 
of  Grapes  partially  de- 
stroyed by  the  Dowiiy 
Mildew.  From  Fartner's 
Bulletin  284,  U.  S.  Dept. 
of  Agriculture. 


Fig.  307.  —  The  haustoria  of  the 
Downy  Mildew  reaching  into  the 
cells  of  the  grape,  h,  hypha;  a, 
haustoria.  From  Bulletin  2H,  Ohio 
Agr.  Exp.  Sta. 


furnished  by  the  leaves  and  as  a  result  both  fruit  and  vine 
may  suffer.  Often  the  fruit  is  directly  attacked  and  de- 
stroyed. After  the  Mildew  is  well  established  within  the  tissues 
of  the  hosts,  it  sends  through  the  stomata  numerous  branches 
which  constitute  the  superficial  downy  patches  characteristic 
of  the  parasite  {Fig.  308).  On  the  tips  of  these  protruding 
hyphae  are  produced  small  globular  bodies  known  as  conidio- 
spores  or  conidia,  and  the  hyphae  bearing  them  are  called 
conidiophores  which  means  "conidia  bearing." 

The  conidia  are  really  small  sporangia  which  break  off  and  are 


POTATO  BLIGHT   (PHYTOPHTHORA   INFESTANS)         357 


scattered  about  like  spores.  When  the  conidia  germinate, 
instead  of  producing  hyphae  they  produce  zoospores,  which,  after 
swimming  about  for  a  few  minutes,  lose  their  ciha  and  begin  to 
produce  new  hyphae.  If  favorably  located,  the  new  hyphae  find 
entrance  to  a  leaf  through  its  stomata  and  start  the  disease  anew. 
The  oogonia  and  antheridia  resemble  those  of  Saprolegnia,  but 
are  produced  on  short  hyphae 
within  the  tissues  of  the  host. 
The  oospore  has  a  heavy  wall 
and  is  not  liberated  until  the 
tissues  of  the  host  surrounding 
it  decay.  The  oospores  are  well 
fitted  to  endure  winter  condi- 
tions, and  as  the  dead  leaves 
are  scattered,  the  oospores  con- 
tained are  also  scattered,  and 
when  freed  it  is  probable  that 
they  often  start  the  disease  the 
following  year. 

Potato  Blight!  (Phytophthora 
infestans).  —  This  Fungus,  com- 
monly called  the  Late  Blight  of 
the  Potato,  is  a  near  relative  of 
the  Grape  Mildew.  It  attacks 
the  leaves,  stems,  and  tubers 
of  the  Irish  Potato  and  is  very 
rapid  and  destructive  in  its 
work.  Figure  309  shows  the 
leaves  of  a  Potato  plant  affected 
with    this    disease.      Like     the 

Grape  Mildew,  after  the  mycelium  is  well  established  in  the 
host,  conidiophores  are  produced  (Fig.  310).  The  conidia  may 
grow   directly   into    hyphae  or   produce    zoospores    {Fig.   311). 

1  Late  Blight  and  Rot  of  Potatoes.  Circular  19,  Cornell  University  Agr 
Exp.  Sta. 

Investigations  of  the  Potato  Fungus,  Phjiiophthora  Infestans.  Bullelin  168, 
Vermont  Agri.  Exp.  Sta. 

Gennination  and  Infection  with  the  Fungus  of  the  Late  Blight  of  Potato 
Research  Bulletin  37,  Wisconsin  Agr.  Exp.  Sta.,  1915. 

Studies  of  the  Genus  Phytophthora.  Vol.  8,  No.  7,  pp.  233-276,  Jour 
Agr.  Research,  U.  S.  Dept.  Agr.,  1917. 


Fig.  308.  —  Reproduction  in  the 
Downy  Mildew  of  the  Grape,  a, 
conidiophores  bearing  conidiospores 
on  the  ends  of  their  branches;  b, 
conidiospores;  c,  oospore;  z,  zo- 
ospore. Much  enlarged.  From 
Farmer's  Bulletin  284,  U.  S.  Dept. 
of  Agriculture. 


358 


THALLOPHYTES 


The  conidia  and  zoospores  which  they  produce  spread  the 
disease  very  rapidly  in  moist  weather.  Since  the  zoospore 
is  a  swimmer,  it  can  function  more  efficiently  during  moist 
weather.  Moist  weather  also  favors  the  germination  of  the 
conidia.  Little  is  known  about  the  sex  organs  of  the  Potato 
Blight,  but  in  the  form  which  causes  the  Bean  Blight,  the  sex 


Fig.  309.  —  Leaf  of  Irish  Potato  affected  with  the  Late  BUght.     From 
Bulletin  I40,  Cornell  University. 


organs  and  oospores  occur  in  the  seed  coat  or  cotyledons  of  the 
seed,  in  which  case  the  oospore  is  planted  with  the  seed.  In  the 
Phytophthora  cactorum,^  which  is  destructive  to  Ginseng,  the  sex 
organs  and  oospores  have  been  found  in  the  stem  and  roots 
{Fig.  312). 

There  are  many  Downy  Mildews  which  give  us  trouble.  In 
fact  many  of  our  plants  such  as  Cucumbers,  Melons,  Beans, 
Potatoes,  Lettuce,  Grapes,  etc.,  are  attacked  and  much  damaged 
by  Downy  Mildews.  But  the  study  of  the  Grape  Mildew  and 
Potato  Blight  has  given  a  general  knowledge  of  their  habits.  In 
combatting  the  Mildews  one  must  reckon  with  conidiospores, 
zoospores,  and  oospores. 

One  is  able  to  check  the  spread  of  the  disease  by  spraying  the 
plants  with  a  solution  that  is  poisonous  to  conidia  and  zoospores. 


Phytophthora  Disease  of  Ginseng.     Bulletin  363,  Cornell  Agr.  Exp.  Sta. 


1915. 


POTATO  BLIGHT   (PHYTOPHTHORA   INFESTANS)         359 


A  spray  that  is  very  commonly  used  is  Bordeaux  mixture.^  The 
oospores  live  over  winter  and  may  perpetuate  the  disease  from 
year  to  year.  Portions  of  diseased  plants  containing  oospores, 
when  hauled  out  in  manure  or  scattered  about  l)y  the  wind,  may 
be  a  means  of  spreading  the  disease. 

In  some  forms  of  the  Peronosporales  as  in  Albugo  or  White 
Rust,  which  forms  white  blisters  on 
the  leaves  and  stem  of  the  Radish 
and  other  plants  of  the  Mustard 
family,  both  the  sex  organs  and 
conidiospores  are  produced  internally. 
The  hj'phae  form  in  clusters  under 
the  epidermis  and  form  conidiospores 
in  chains  which  push  up  the  epider- 
mis, forming  white  blisters  which 
finally  rupture  and  allow  the  spores 
to  escape.  In  this  Fungus  the 
conidiospore  produces  a  number  of 
zoospores. 

In  this  order  Pythium  is  some- 
times included,  species  of  which  at- 
tack seedlings  in  greenhouses,  causing 
the  rapid  wilting  known  as  damping 
off,  when  moisture  and  warmth  are 
abundant.  Some  species  of  Pythium 
live  in  the  water  like  the  Saproleg- 
niales  in  which  order  Pythium  is 
often  put,  while  other  species  live  in 
the  soil. 

In  contrast  to  the  Water  Molds, 
the  Downy  Mildews  are  chieflj^  para- 
sitic, much  less  aquatic  and,  having  introduced  the  conidia,  they 
depend  less  upon  water  for  dissemination.     But  like  the  Water 
Molds  the  presence  of  zoospores  and  the  character  of  the  re- 
productive organs  suggest  a  relationship  to  the  Green  Algae. 


Fig.  310.— The  lower 
epidermis  of  a  Potato  leaf 
showing  the  conidiophores  of 
the  Late  Bhght  protruding 
through  the  stomata  and 
bearing  conidiospores  at  the 
tips  of  their  branches.  Many- 
times  enlarged. 


'  The  preparation  as  most  commonly  made  consists  of  5  pounds  of  copper 
sulphate  and  5  pounds  of  stone  lime  dissolved  in  50  gallons  of  water.  Potato 
Spraying  Experiments  in  1906.  Bulletin  279,  New  York  Agr.  Exp.  Sta.  Cer- 
tain Potato  Diseases  and  their  Remedies.  Bulletin  72,  Vermont  Agr.  Exp. 
Sta.,  1899. 


360 


THALLOPHYTES 


True  Molds  (Mucorales).  —  There  are  a  number  of  Molds 
some  of  which  belong  to  other  divisions  of  the  Fungi.  The 
Molds  of  this  order  are  characterized  by  a  zygosporic  reproduc- 
tion, on  which  account  they  are  called  Zygomijcetes.  Of  the 
nearly  200  species  known,  Bread  Mold  is  the  most  familiar  one. 


Fig.  311.  —  Conidia  of  the  Late  Blight  of  the  Potato  developing 

and  zoospores  growing  hyphae.     X  about  400.     After  Ward 


Bread  Mold  (Rhizopus  nigricans).  —  Bread  Mold  is  very 
common  about  homes,  producing  a  fluffy  tangle  of  hyphae  on  the 
surface  of  bread,  fruit,  and  other  favorable  nutrient  substances 
when  left  exposed  {Fig.  313).  It  is  sometimes  injurious  to 
Sweet  Potatoes  and  other  vegetables  in  storage.  The  fluffy 
tangle  of  hyphae  is  white  while  young  but  becomes  dark  when  old, 
owing  to  the  dark  color  of  the  mature  sporangia. 

A  strong  poison  has  been  found  in  connection  with  Rhizopus 
nigricans,  and  it  has  been  suggested  that  some  of  the  diseases  of 
stock,  such  as  the  "cornstalk  disease"  and  the  "horse  disease," 
prevalent  in  some  of  the  Western  states,  may  be  due  to  the  toxin 
which  stock  get  in  moldy  fodder  or  other  feed.  The  toxin 
apparently  is  only  effective  when  introduced  into  the  circulatory 
system.  This  is  shown  by  the  fact  that  rabbits  can  be  fed  the 
Mold  without  any  injury,  but  when  a  little  of  the  sap  is  expressed 
from  the  mycelium  and  injected  into  the  blood,  the  animal  dies 
almost  instantly. 

The  mycelium  consists  of  numerous  coenocytic  branching 
hyphae.  Some  of  the  hyphae  penetrate  the  substratum  and 
gather  food,  while  others  grow  above  the  substratum  and  produce 
the  visible  fluffy  mass.     The  surface  hyphae  with  more  or  less 


BREAD  MOLD    (RHIZOPUS  NIGRICANS) 


361 


upright  growth  bear  the  sporangia,  while  others  running  over 
the  surface  of  the  substratum  produce  at  certain  places  a  new 
set  of  both  penetrating  and  upright  hyphae.  These  runner-hke 
hyphae  are  called  stolons,  and  serve  to  spread  the  mycelium  over 
the  substratum.  The  hyphae  which  penetrate  the  substratum 
are  able  to  change  the  elements 
of  the  substratum  into  soluble 
forms  and  absorb  them. 

The  sporangia  occur  singly  on 
the  hyphae  and  contain  numer- 
ous aerial  spores,  which  when 
mature  are  liberated  by  the 
breaking  of  the  sporangial  wall. 
The  spores  are  nearly  always 
present,  floating  about  in  the  air 
and  resting  on  objects  where 
they  happen  to  fall.  It  is  prob- 
able that  they  can  live  for  many 
years  in  the  dormant  state  and 
then  germinate  when  they  come 
in  contact  with  suitable  food 
material. 

The  Bread  Mold  has  no  sex 
organs,  but  there  is  a  sexual 
process  which  reminds  one  of 
the  sexual  process  in  Spirogyra. 
Sometimes,  as  shown  in  Figure 
314,  tips  of  hyphae  approach 
each  other  and  finally  meet.  From  each  hyphae  an  end  cell  is  cut 
off,  and  these  end  cells  fuse  to  form  heavy  walled  zygospores. 
Upon  germination  the  zj^gospore  produces  an  erect  hj-pha  bearing 
a  sporangium  of  the  ordinary  type,  and  the  aerial  spores  developed 
therein  are  capable  of  starting  a  new  series  of  plants. 

Conjugation  is  only  occasionally  obtained  in  Rhizopus  nigricans 
unless  the  cultures  are  made  in  a  certain  way.  It  has  been  found 
that  in  Rhizopus  nigricans  there  are  two  kinds  of  plants,  which, 
although  looking  just  alike,  behave  differently.  They  are  called 
strains,  one  being  known  as  the  plus  (+)  and  the  other  as  the 
minus  (  — )  strain.  When  either  of  these  occur  alone  in  a  culture 
then  no  conjugation  takes  place,  but  if  both  are  present  then 


-  Methods  of  repro- 
duction in  the  Phytophthora  cacto- 
rum,  which  attacks  Ginseng.  A,  sex 
organs  consisting  of  oogonium  (o) 
and  antheridium  (a).  B,  conidi- 
ospore  forming  zoospores  above, 
and  a  group  of  zoospores  below. 
C,  conidiospore  producing  hyphae 
directly.  Much  enlarged.  From 
Bulletin  363,  Cornell  University 
Agr.  Exp.  Sta. 


362 


THALLOPHYTES 


there  is  abundant  conjugation  and  formation  of  zygospores.  In 
many  laboratories  the  spores  of  both  strains  are  kept  in  stock, 
and  conjugation  is  obtained  whenever  desired  by  using  spores 
of  both  strains  in  growing  the  cultures. 

Another   Mold   of  this  order  is  Pilobolus,   commonly  called 
Squirting  Fungus  on  account  of  the  way  it  throws  its  sporangia. 


Fig.  313.  —  Bread  Mold,  Rhizopus  nigricans.  A,  piece  of  bread  on  which 
there  is  a  growth  of  Mold  (X  5).  B,  plant  body  of  Bread  Mold,  showing 
the  hyphae  (r)  which  penetrate  the  bread,  the  hyphae  which  grow  up  and  bear 
the  sporangia  (s),  and  the  hyphae  (a)  (stolons)  which  grow  prostrate  on  the 
surface  of  the  substratum  and  start  new  plants.     (X  about  20.) 

It  is  common  on  stable  manure  and  resembles  Bread  Mold.  The 
hyphae  become  turgid  and  swollen  just  beneath  the  sporangia 
and  finally  burst,  hurling  the  sporangia  with  considerable  force, 
whence  the  name  Squirting  Fungus. 

In  the  True  Molds,  where  there  are  no  swimming  spores,  the 
Phycomycetes  become  entirely  aerial,  although  the  coenocytic 
plant  body  and  conjugation  still  suggest  a  relationship  with  the 
Green  Algae.  The  mycelium,  a  tangle  of  hyphae  with  no  definite 
shape  in  Phycomycetes,  shows  some  differentiation  into  absorbing, 
vegetative,  and  reproductive  structures.  The  chief  propagative 
structures  of  the  group  are  zoospores,  conidia,  and  aerial  spores. 


ASCOMYCETES 


363 


Oospores  and  zygospor(;s  tide  the  plant  over  unfavorable  con- 
ditions and  produce  new  plants  when  favorable  conditions  return. 
In  combatting  the  disease-producing  Phycomycetes,  the  control 
of  zoospores,  conidia,  and  oospores  must  be  considered. 


Fig.  314.  —  Conjugation  in  Bread  Mold.  «,  b,  c,  and  d  are  successive 
stages  in  conjugation.  At  a  the  short  hyphae  have  just  come  together,  wliile 
at  d  the  zygospore  is  formed,  e,  zygospore  developing  a  new  hypha  bearing 
a  sporangium.      X  about  130. 


Ascomycetes  (Sac  Fungi  and  Lichens) 

General  Description.  —  To  the  Ascomycetes  belong  the  largest 
number  of  Fungi,  and  most  of  them  are  parasites.  Many  of  our 
most  troublesome  diseases  are  caused  by  these  Fungi.     Some, 


364  THALLOPHYTES 

like  the  Yeast  Plant,  and  the  Molds  which  help  in  making  cheese, 
are  useful.  Some  of  the  Ascomj'cetes  are  used  directly  as  food. 
The  saprophj'tic  forms  are  useful  in  hastening  the  decay  of  or- 
ganic matter.  But  the  main  reason  for  their  study  is  the  desire 
to  be  able  to  stop  the  destruction  caused  by  the  disease-producing 
forms. 

The  Ascomycetes  are  so  named  because  of  the  ascus  or  sac 
which  is  the  characteristic  spore-bearing  structure  of  the  group. 
The  ascus  is  an  enlarged  end  of  a  hypha  which  becomes  a  thin 
walled  sac  in  which  spores  are  produced.  Any  Fungus  producing 
spores  in  an  ascus  is  called  an  Ascomycete.  The  spores  produced 
in  an  ascus  are  called  ascospores.  The  Ascomycetes  have  other 
spores,  but  the  ascospores  are  the  most  general  ones. 

The  Ascomycetes  differ  from  the  Phycomycetes  in  having  no 
zoospores  and  in  having  hyphae  divided  by  cross  walls.  Many  of 
the  Ascomycetes  have  sex  organs  and  differentiated  gametes,  but 
the  cell  resulting  from  fusion  develops  immediately  into  asci,  so 
there  are  no  resting  oospores  to  be  considered  in  this  group. 
Taking  care  of  the  ascospores  takes  care  of  the  results  of  fertili- 
zation. 

The  Ascomycetes  vary  widely  in  character  of  plant  body  and 
methods  of  reproduction.  In  some  the  plant  body  is  a  structure 
with  a  definite  form,  while  in  others  it  is  only  a  scattered  mass  of 
hyphae.  In  some  the  plant  body  is  very  prominent,  but  extremely 
inconspicuous  in  others.  Some  have  well-defined  sex  organs, 
while  others  apparently  have  abandoned  sexual  reproduction  and 
have  lost  their  sex  organs.  Their  sex  organs  resemble  those  of 
the  Red  Algae  and  this  is  the  feature  that  suggests  their  relation- 
ship to  the  Algae.  There  are  about  15  orders  and  29,000  species 
of  Ascomycetes.  The  Morels  (Helvellales),  Cup  Fungi  (Pezizales), 
Closed  Fungi  (Pyrenomycetales) ,  Naked  Ascus  Fungi  (Protodi 
scales),  Mildews  {Peri  sporiales),  the  Blue  and  Green  Molds 
(Plectascales) ,  and  the  Yeasts  (Protoascales)  are  familiar  orders. 

The  Morels  (Helvellales).  —  Not  all  of  the  Fungi  of  this  order 
are  Morels,  but  the  Morels  are  the  most  familiar  ones.  The 
fleshy  plant  body  with  a  definite  form  and  often  so  large  as  to 
be  quite  conspicuous  is  one  of  the  notable  features  of  the  Hel- 
vellales. They  are  mostly  saprophytic  and  the  mycelium  usually 
develops  underground  where  it  lives  on  decaying  wood,  leaves, 
etc.     Here  belongs  the  Edible  Morel  shown  on  next  page. 


COMMON  EDIBLE   MOREL   (MORCHELLA   ESCULANTA)      365 

The  Common  Edible  Morel  (Morchella  esculentaj.  —  The 
common  Edible  Morel  is  found  in  the  spring,  commonly  in  j\Iuy 
and  early  June.  It  is  quite  generally  collected  and  used  for  food. 
It  is  often  called  a  Mushroom,  although  it  is  not  the  cultivated 
Mushroom.  Morels  are  usually  found  in  the  woods  among  the 
leaves  and  about  old  logs  and  stumps.  Often  they  grow  in 
clusters  as  Figure  315  shows.  The  wrinkled  top  and  supporting 
stalk  consist  of  hyphae  so  massed  together  as  to  form  a  definitely 


Fig.  3L5.  —  A  cluster  of  Morels,  Morchella  esculenta  (X  5).     Photographed 
by  C.  M.  King. 

shaped  plant  body.  The  mycelium  absorbs  food  from  decaying 
organic  matter  in  the  earth,  and  when  it  is  well  established  in  the 
soil,  the  portion  above  ground  is  produced.  The  asci  with  the 
ascospores  are  produced  in  the  pits  of  the  wrinkhnl  top  which  is 
known  as  the  ascocarp.  A  small  portion  of  a  section  through  a 
pit,  as  seen  under  the  microscope,  is  shown  in  Figure  316.  The 
asci  are  numerous  and  each  contains  eight  ascospores.  The  asci 
with  the  intermingling  sterile  hyphae,  called  paraphyses,  consti- 
tute a  distinct  layer,  known  as  the  hymenium,  on  the  surface  of 
the  ascocarp.  After  the  spores  are  mature,  the  ascocarp  decays 
and  frees  the  spores  which  are  widely  distributed  by  wind  and 


366 


THALLOPHYTES 


other  agents.     When  located  on  favorable  organic  matter,  the 
spores  grow  directly  into  new  mycelia. 

Although   Morels  spring  up   quickly,   often  apparently  over 
night,  much  time  is  required  for  the  development  of  the  sub- 
terranean mycelimn  before  the  aerial  portion  is  developed.      No 
sexual  reproduction  has  been  discovered 
in  the  Morels,  and  the  only  spore  known 
is  the  ascospore. 

Some  other  edible  Ascomycetes,  which 
command  high  prices  in  Europe,  are  the 
Truffles,  which  belong  in  the  order  Tube- 
rales.  The  distinctive  feature  of  the 
Truffles  is  that  the  ascocarp  occurs 
wholly  underground.  The  ascocarp, 
which  is  tuber-like,  is  closed  except 
for  a  small  opening  and  the  spores  are 
released  by  the  decay  of  its  walls.  Since 
they  are  underground,  they  are  very 
difficult  to  find,  and  experts  hunt  them 
by  the  aid  of  trained  pigs  or  dogs  which 
detect  them  through  the  sense  of  smell. 
No  sexual  reproduction  has  been  dis- 
covered, but  not  much  is  known  of  their 
life  cycle. 

Cup  Fungi  (Pezizales).  —  The  Cup 
Fungi  include  many  species  most  of 
which  are  saprophytes.  The  loose  my- 
celium develops  in  decaying  rich  humus,  decaying  wood,  or  leaf 
mold,  and  when  well  established  it  produces  above  the  surface  an 
ascocarp  which  has  the  form  of  a  disk,  funnel,  or  cup.  Such  an 
ascocarp  is  called  an  apothecium  to  distinguish  it  from  other  types 
of  ascocarps. 

Peziza.  —  This  genus,  a  species  of  which  is  shown  in  Figure 
317,  is  common  in  the  woods  and  the  cup-shaped  apothecium  is 
sometimes  2  or  3  inches  across  and  often  brightly  colored.  In 
one  common  form  the  interior  of  the  cup  is  bright  scarlet.  The 
interior  of  the  apothecium  is  lined  with  a  hymenium  consisting 
of  parallel,  sterile,  hyphal  threads  or  paraphyses  among  which 
occur  the  asci  each  containing  eight  spores.  By  the  swelling  and 
ruptiu'ing  of  the  asci  the  ripe  spores  are  expelled  and  then  scat- 


FiG.  316.  — Asci  (a)  of 
the  Morel,  showing  the 
ascospores  ( X  about  200) . 
The  hyplia  (p)  producing 
no  spores  is  called  a  pa- 
raphysis. 


PYRONEMA 


367 


tered  by  the  wind.  No  sexual  reproduction  has  been  tliscovered 
in  Peziza,  but  in  Pyronema,  a  form  similar  to  Peziza,  sexual 
reproduction  has  been 
discovered  and  carefully 
followed. 

Pyronema.  —  In  this 
form  there  are  sex  organs 
and  the  apothecium  de- 
velops as  a  result  of 
fertihzation  {Fig.  3 IS). 
The  female  sex  organ 
resembles  that  of  the 
simpler  Red  Algae,  such 

as  Nemalion.     It  consists  of  a  globular  cell  (oogonium)  and  an 
elongated  tube-like  cell  (trichogyne  or  conjugating  tube).     The 


Fig.  317.  —  A  cluster  of  Cup  Fungi, 
Pezizas.      X  h- 


Fig.  318.  —  Sexual  reproduction  in  Pyronema  confluans.  A,  the  sex  organs 
at  the  time  of  fertilization,  showing  the  autheridia  (a)  in  contact  and  fusing 
with  the  trichogynes  through  which  the  sperms  pass  to  the  oogonia  (o) ;  B,  de- 
V(>lopment  of  apothecium,  showing  the  oogonia  developing  ascogenous  hyphae 
which  are  beginning  to  form  asci  at  the  ends  of  their  branches,  and  the  sterile 
hyphae  (6)  which  grow  up  among  the  ascogenous  hyphae  and  form  a  large 
part  of  the  wall  of  the  apothecium.     Highly  magnified.     After  Harper. 


antheridium  is  a  somewhat  club-shaped  terminal  cell  which 
comes  in  contact  with  the  tip  of  the  trichogyne  and  fuses  with 
it.     Both  oogonium  and  antheridium  are  multinucleate.       The 


368 


THALLOPHYTES 


numerous  nuclei  of  the  antlieiidiuni  flow  into  the  trichogyne  and 
pass  on  into  the  oogonium  whow  tlio}^  pair  and  fuse  with  the 
numerous  nuclei  of  the  oogonium.  From  the  fertilized  oogonium, 
now  known  as  the  ascogoniuni,  branches  called  ascogenous  hyphae 
arc  developed  and  on  the  ultimate  branches  of  these  are  produced 

the  asci.  From  beneath  the 
ascogoniuni  sterile  hyphae 
(hyphae  producing  no  asci) 
grow  up  among  the  ascoge- 
nous hyphae  and  constitute  the 
paraphyses  of  the  hymenium. 
Other  sterile  hyphae  form  the 
wall  of  the  cup-shaped  plant 
■'^^"JR^'^M^  ""'  <^  i^?^  body  or  ascoearp.  Usually 
several  oogonia  are  involved 
in  the  formation  of  a  single 


ascoearp. 

Brown  Rot  of  Stone  Fruits 
(Sclerotinia      fructigena).  — 

This  Fungus,  shown  in  Figure 
319,  is  one  of  the  parasitic 
forms  of  the  Pezizales.  In 
some  years  this  Fungus  is  an 
extremely  destructive  para- 
site. It  attacks  nearly  all 
stone  fruits  and  in  some  years 
nearly  half  of  the  Plum  and 
Peach  crop  may  be  destroyed 
by  this  disease.  In  Georgia 
the  estimated  loss  in  Peaches 
and  Plums  caused  by  this 
disease  in  1900  was  between 
$500,000  and  $700,000.  To  a 
limited  extent  it  attacks  the  twigs  and  flowers  and  does  some 
damage  in  this  way. 

Fruits  half  size  or  larger  seem  to  be  most  susceptible  to  the 
attack  of  the  Fungus.  The  disease  first  shoAVs  as  small  decayed 
spots,  dark  brown  in  color.  The  fruit  decays  rapidly  and  soon 
hyphae  break  through  from  beneath,  forming  moldy  patches  on 
the  surface.     The  moldy  patches  contain  conidiophores  which 


Fig.  319.  —  Sclerotinia  fructigena. 
Above,  the  apothecia  developed  on  a 
decayed  Plum;  at  the  right,  below, 
section  through  an  apothecium,  show- 
ing asci  and  paraphyses;  at  the  left, 
below,  an  ascus  and  paraph  ysis  more 
highly  magnified.     After  Duggar. 


BLACK   KNOT   (PLOWRIGHTIA   MORBOSA)  369 

produce  conidiosporcs  abundantly.  The  conidiosporcs  can  live 
over  till  the  succeeding  season  and  start  the  disease  anew.  The 
disease  is  propagated  chiefly  by  conidiospores.  It  was  a  long 
time  after  the  disease  was  known  before  ascospores  were  found 
and  of  course  it  was  not  then  classed  as  an  Ascomycete  but  was 
put  into  the  class  Fungi  Imperfecti.  Apparently  ascospores  are 
often  not  formed  at  all,  and,  when  they  are,  they  occur  in  the 
diseased  fruits  after  they  have  dried  up  and  usually  fallen  from 
the  tree.  As  the  fruit  decays  it  dries  up  into  a  mummy.  In  this 
dried-up  fruit,  regardless  of  whether  it  is  on  the  ground  or  on  the 
tree,  the  mycelium  becomes  changed  into  compact  masses  called 
sclerotia.  Later,  probably  the  next  spring,  upon  these  sclerotia 
are  developed  bell-shaped  apothecia  in  which  the  ascospores  occur 
(Fig.  319).  Thus  in  controlling  the  disease  the  destruction  of 
the  mummied  fruits  as  well  as  spraying  to  kill  the  conidiospores 
that  are  sticking  to  the  buds  and  bark  are  advised. 

The'  Closed  or  Black  Fungi  (Pyrenomycetales).  —  These 
Fungi,  of  which  there  are  about  11,000  species,  include  both 
parasites  and  saprophytes.  They  vary  much  in  form  and 
manner  of  growth.  They  are  chiefly  characterized  by  a  super- 
ficial, compact,  black  mycelium  looking  as  if  it  had  been  charred 
by  fire.  The  structure  in  which  the  asci  are  produced  is  a  peri- 
thecium,  a  small  commonly  flask-shaped  cavity  with  a  small 
pore-like  opening.  Many  of  these  Fungi  produce  destructive 
plant  diseases,  of  which  the  Black  Knot,  Ergot,  and  Chestnut 
Disease  are  familiar  ones. 

Black  Knot  (Plowrightia  morbosa).  —  This  Fungus  occurs  on 
the  twigs  of  Plum  and  Cherry  trees,  producing  wart-like  excres- 
cences as  shown  at  A  in  Figure  320.  The  mycelium  attacks  the 
cambium,  phloem,  and  cortex,  causing  at  first  an  abnormal  growth 
and  later  the  death  of  these  tissues.  As  a  result  of  the  attack,  the 
twig  is  much  injured  or  killed.  The  attack  is  often  so  general  that 
the  entire  tree  is  killed.  The  wart-Hke  excrescences  or  knots  con- 
sist of  the  mycelium  and  the  abnormally  developed  tissues  of  the 
host.  During  the  first  summer  the  disease  shows  as  slight  swell- 
ings, but  with  the  renewed  growth  of  the  following  spring,  the 
swellings  enlarge  rapidly,  and  during  May  or  June  the  mycelium 
breaks  through  the  bark  and  forms  a  dense  covering  over  the  sur- 
face of  the  swellings.  From  the  hyi^hae  forming  the  covering  of 
the  knot  numerous  erect  hyphae  arise  which  give  the  knot  a 


370 


THALLOPHYTES 


velvety  appearance.  These  erect  hyphae  are  conidiophores  and 
bear  conidiospores  as  shown  at  B  in  Figure  320.  The  conidi- 
ospores  are  scattered  by  the  wind  and  upon  germination  grow 
directly  into  hyphae  which  can  penetrate  a  young  shoot  and  start 

the  disease  anew.  In  late 
summer  after  the  produc- 
tion of  conidiospores  is 
over,  the  knot  becomes 
black  and  on  its  surface 
occur  numerous  small 
papillae  which  are  the 
flask  -  shaped  perithecia, 
openmg  with  a  pore  and 
Hned  on  the  inside  with 
asci  as  shown  at  C  n 
Figw^e  320.  The  asco- 
spores  are  mature  and 
ready  to  be  distributed 
early  the  next  spring. 

It  follows  then  that  the 
disease  may  be  spread  dur- 
ing the  early  spring  by 
ascospores  or  during  late 
spring  and  summer  by  the 
conidiospores.  The  de- 
struction of  the  knots  be- 
fore the  shedding  of  the 
spores  will  check  the  dis- 
ease. Bordeaux  mixture 
applied  at  proper  times  is 
useful  in  checking  the  dis- 
ease, but  most  attention 
should  be  given  to  the  de- 
struction of  the  diseased 
branches. 


Fig. 


D 

320.  —  Black  Ivnot,  Plowrightia 
morbosa.  A,  branch  of  a  Plum,  showing  the 
wart -like  excrescences  caused  by  the  Fungus; 
B,  conidiophores  producing  conidiospores 
(X  500),  and  at  the  right  a  conidiospore 
germinating;  C,  two  perithecia  sectioned 
lengthwise,  showing  the  asci  and  paraphyses 
within  ( X  50) ;  D,  asci  and  paraphyses  more 
highly  magnified. 


Ergot  (Claviceps  purpurea  and  Paspali).^  —  Ergot  is  a  parasite 
on  the  young  ovaries  of  the  Grasses,  being  especially  common  on 
Rye  and  occurring  sometimes  on  Wheat,  Barley,  and  a  number  of 


'  Ergot  and  Ergotism.  Press  Bulletin  23,  Nebraska  Agr.  Exp.  Sta.,  1906. 
Life  History  and  Poisonous  Properties  of  Claviceps  Paspali.  Vol.  7,  No.  9, 
pp.  401-406,  Jour.  Agr.  Research,  U.  S.  Dept.  Agr.,  1916. 


THE   CHESTNUT   DISEASE   (ENDOTHIA  PARASITICA)      371 


other  Grasses.  The  ascospores  affect  the  ovaries  in  early  suiriiner. 
In  the  ovary  the  myceUum  develops,  using  the  food  material 
which  the  ovary  should  have.  The  mycelium  produces  on  the 
surface  of  the  ovary  numer- 
ous conidiophores  which 
produce  conidia  abundantly, 
and  the  conidia  are  dis- 
seminated largely  by  insects 
which  seek  the  honey  dew 
secreted  by  the  mycelium. 
After  the  tissues  of  the 
ovary  are  destroyed,  the 
mycelium  becomes  trans- 
formed into  a  dark,  hard, 
club-shaped  body  called 
sclerotium  which  projects 
from  the  spikelet  as  shown 
in  Figure  321.  These 
bodies,  which  are  the  so- 
called  Ergot,  contain  one  or 
more  alkaloids  which  are 
poisonous  to  both  man  and 
live  stock.  Stock  are  some- 
times badly  poisoned  by 
eating  Timothy,  Red  Top, 
and  other  kinds  of  hay 
where  Ergot  is  abundant. 
The  sclerotia  fall  to  the 
ground  and  pass  the  winter. 
The  next  spring  they  de- 
velop branches  which  bear 
rose-colored  globular  heads, 
called  stromata,  in  which  the 
asci  are  produced  in  sunken 


Fig.  321.  —  The  Ergot  Fuiigus,  Cluri- 
ceps  purpurea,  a,  head  of  Rye,  showing 
projecting  sclerotia;  b,  a  sclerotium  which 
has  developed  stalks  bearing  globular 
heads  in  which  the  perithecia  occur  ( X  3) ; 
c,  section  through  one  of  the  globular 
heads,  showing  the  perithecia  (X  15);  d, 
ascus  highly  magnified,  showing  the 
spindle-shaped  ascospores;  e,  hypha  and 
conidia  which  develop  on  the  surface  of 
the  grain  in  the  early  stage  of  infection. 
From  Tulasne  and  Strasburger. 


perithecia. 

The  Chestnut  Disease  (Endothia  parasitica). — This  disease 
was  introduced  from  Asia  and  appeared  in  New  York  about 
1904.  It  is  very  destructive  to  Chestnut  trees,  and  the  estimated 
loss  in  New  York  City  and  vicinity  is  more  than  $5,000,000. 
For  the  entire  United  States,  the  financial  loss  up  to  1911  was 


372 


THALLOPHYTES 


estimated  at  about  $25,000,000.     So  serious  is  this  disease  that 
legishitures  have  made  special  appropriations  for  fighting  it. 

The  spores  are  carried  by  the  wind  and  sometimes  by  birds 
and  insects.  When  the  spores  reach  the  bark  of  the  Chestnut, 
they  develop  hyphae  which  penetrate  and  kill  the  phloem  and 
cambium.  The  dead  bark  soon  becomes  warty  with  yellowish- 
brown  pustules  in  which  summer  spores  in  great  numbers  are 


Fig.  322. —  Pus- 
tules on  the  bark  of 
a  Chestnut  caused 
by  the  Chestnut 
Blight  Fungus. 
PVom  Bullelin  380, 
U.  S.  Dept.  Agri- 
culture, 1917. 


Fig.  323.  —  Powdery  Mildew 
on  an  Apple  leaf.  The  light 
areas  are  due  to  the  presence 
of  many  superficial  hyphae. 
From  Bulletin  185,  Maine  Agr. 
Exp.  Sta. 


produced  {Fig.  322).  The  summer  spores  are  extruded  in 
threads  and  spread  the  disease  to  other  trees.  In  autumn  these 
same  pustules  develop  deeply  buried  perithecia  in  Avhich  the 
ascospores  (winter  spores)  develop.  The  ascospores  germinate 
the  next  spring  and  when  carried  to  other  trees  start  the  disease 
anew.  The  mycelium  in  an  affected  tree  renews  its  activity  each 
year  and  thus  continues  to  spread,  usually  downward,  until  the 


POWDERY   MILDEWS   (PERISPORIALES)  373 

tree  is  killed.  The  deeply  buried  mycelium  is  not  reached  by 
sprays,  and  the  total  destruction  of  the  infected  trees  is  the  only 
available  method  of  checking  the  disease. 

Powdery  Mildews  (Perisporiales). — This  group  includes 
many  Fungi,  but  they  are  all  very  similar  in  their  habits.  The 
mycelium  commonly  occurs  on  the  surface  of  leaves,  but  some- 
times on  the  stems  and  fruits  of  the  higher  plants.     The  myce- 


FiG.  324.  —  Powdery  Mildew  of  the  Hop.  Below,  diagrammatic  draw- 
ing of  a  section  of  a  Hop  leaf,  showing  the  superficial  mycelimn  which  has 
grown  haustoria  into  the  epidermal  cells,  and  produced  erect  conidiophores 
bearing  chains  of  conidia  (X  about  50).  Above,  epidermal  cell,  hypha,  and 
invading  haustorium  more  highly  magnified.  From  Bulletin  328,  Cornell 
University  Agr.  Exp.  Sta. 

lium  forms  quite  noticeable  powdery  patches.  The  asci  are 
produced  in  closed  ascocarps  called  cleistothecia.  In  Figure  323 
is  shown  the  mildew  of  the  Apple. 

The  Lilac  Mildew  {Microsphaera)  is  the  one  most  commonly 
observed  of  the  Mildews.  Often  in  late  summer  and  autumn, 
the  leaves  of  the  Lilac  are  so  generally  covered  with  the  whitish 
dusty-looking  patches,  that  the  entire  bush  appears  covered  with 
street  dust.  But  there  are  also  Mildews  that  occur  on  fruit 
trees,  Roses,  Gooseberries,  Peas,  and  other  cultivated  plants, 
which  do  considerable  damage.      From  the  superficial  hyphae 


374 


THALLOPHYTES 


haustoria  are  sent  into  the  host.  These  liaustoria  absorb  food 
from  the  tissues,  and  often  cause  consideral)le  injury  to  the 
leaves  and  fruit. 

From  the  superficial  hyphae  arise  numerous  erect  conidio- 
phores,  which  pioduce  chains  of  conidiospores  (Fig.  32Jf).  The 
powdery  appearance  of  the  Fungus  is  due  to  the  ascocarps  and 
the  numerous  conidiospores.  The  conidiospores  are  distributed 
by  the  wind  and,  when  favorably  placed,  grow  directly  into  hyphae, 
and  are  the  means  of  producing  new  growths  of  the  Mildew. 
Late  in  the  summer  and  autumn,  the  superficial  hyphae  form 


Fig.  325.  —  At  the  left,  surface  of  a  leaf  infected  with  Powdery  Mildew, 
showing  the  superficial  mycelium,  ascocarps,  and  conidiophores.  At  the 
right,  a  cleistothecium  broken  open,  showing  the  asci  which  develop  within. 
From  Tulasne  and  Nature. 


globular  heavy-walled  cleistothecia  in  which  the  asci  are  produced 
and  which,  when  mature,  appear  to  the  naked  eye  as  black  dots 
on  the  surface  of  the  leaf  {Fig.  325). 

Projecting  from  the  wall  of  the  ascocarp  are  appendages  which 
may  have  variously  branched  tips.  Enclosed  within  the  heavy 
wall  of  the  ascocarp,  the  ascospores  pass  the  winter.  When  freed 
in  the  spring  by  the  breaking  of  the  ascocarp,  the  spores  may  be 
blown  or  carried  about  and  germinate  upon  a  new  host.  The 
development  of  the  ascocarp  is  a  result  of  fertilization  and  the  sex 
organs,  like  those  of  Pyronema,  suggest  those  of  the  Red  Algae. 

The  ascocarp  of  the  Mildews  suggests  the  cystocarp  of  the 


ASPERGILLUS 


375 


higher  Red  Algae,  such  as  Polysiphonia,  for  as  the  ascogenous 
hyphae  develop  from  the  ascogoniuin,  sterile  hyphae,  growing  up 
from  below  the  ascogonium,  form  a  compact  hard  wall  which 
makes  a  case  for  the  asci  and  ascospores,  just  as  the  filaments 
growing  up  from  below  the 
carpogonium  produce  a  case 
for  the  carpospores  in  Poly- 
siphonia. 

The  Blue  and  Green  Molds 
(Plectascales) .  —  S  u  p  e  r  fi- 
cially  these  Molds  resemble 
the  true  Molds  discussed 
under  the  Mucorales,  but 
their  spore  masses  are  gen- 
erally green  or  blue,  while 
those  of  the  true  Molds  are 
black.  There  are  about  250 
known  species  in  this  order, 
but  they  are  saprophytes  and 
only  a  few  of  them  are  of 
much  importance.  They 
bear  their  ascospores  in 
closed  ascocarps  or  Cleisto- 
thecia.  Aspergillus  and 
Penicillium  are  two  familiar 
genera  of  the  order. 

Aspergillus. — These  Molds 
are  commonly  green  on  ac- 
count of  their  greenish  spore 
masses.  One  form  known 
as  the  Herbarium  Mold  is 
troublesome   in    herbariums 

where  it  attacks  specimens  that  are  not  well  dried, 
often  occur  along  with  the  true  Molds.  They  will  grow 
on  cheese,  leather,  wall  paper,  fruit,  hay,  silage,  and  on 
most  any  damp  object  from  which  they  can  obtain  nourish- 
ment. Some  arc  poisonous  and  stock  are  injured  and 
sometimes  killed  by  eating  them  in  moldy  Corn,  hay,  and 
silage. 

The  loose   extensive  mycelium   runs  over  and   through   the 


Fig.  326.  —  A  species  of  Aspergillus. 
A,  a  portion  of  a  mycelium,  showing  a 
conidiophore  bearing  chains  of  conidia 
(300);  B,  sex  organs  coiled  about  each 
other  and  consisting  of  hyphae  similar 
in  appearance;  C,  the  cleistothecium 
which  develops  after  fertilization  and  in 
which  the  asci  develop  (X  200). 


They 


376 


THALLOPHYTES 


substratum,  and  sends  up  conidiophores  at  the  ends  of  which  the 
conidia  are  borne  in  radiating  chains  as  shown  in  Figure  326. 
The  spores  are  scattered  mostly  by  the  wind. 

The  sex  organs  appear  a  Uttle  later  than  the  conidia  and 
consist  of  two  short  hyphal  filaments  which  come  together  and 
intertwine  spirally.  One  of  these  filaments  represents  the  oogo- 
nium and  the  other,  the  antheridium. 
After  fertilization,  ascogenous  hyphae 
develop  from  the  ascogonium  and  bear 
eight-spored  asci  at  their  tips.  In  the 
meantime  other  hyphae  grow  up  from 
below  the  ascogonium  and  a  closed  case 
or  cleistothecium  is 
formed,  within  which 
are  the  asci  inter- 
mingled amongst 
sterile  hyphae.  The 
walls  of  the  asci 
finally  dissolve,  thus 
setting  the  asco- 
spores  free  within 
the  cleistothecium. 
Through  the  decay 
of  the  wall  of  the  cleistothecium  the  spores  are 
finally  freed  to  be  scattered  by  the  wind. 

Another  Ascomycete  which  sometimes 
poisons  livestock  is  the  Purple  Monascus. 
It  belongs  to  another  order  and  is  a  simpler 
Ascomycete  than  Aspergillus.  It  is  often 
present  in  moldy  silage  and  when  fed  to  live- 
stock may  cause  death.  This  mold  produces 
a  purple  pigment  which  colors  the  substratum 
upon  which  the  mold  lives  and  distinctly  colors 
silage  attacked  by  the  Mold. 

Penicillium.  —  A  common  species  of  Penicillium  is  the  Blue 
Mold  which  develops  on  shoes  or  gloves  left  in  damp  places,  and 
on  lemons,  cheese,  etc.  It  often  occurs  intermingled  with  Bread 
Mold  on  bread.  The  conidia  are  borne  as  shown  in  Figure  327. 
Its  sexual  reproduction  is  similar  to  that  of  Aspergillus  and  the 
clcistothecia   are   about  as    large   as   a   coarse   grain   of   sand. 


Fig.  327.  —  A  species  of 
Penicillium,  showing  conidi- 
ophores bearing  chains  of 
conidia. 


F I G.  3  2  8.  —  A 
naked  -  ascus  Fun- 
gus, Taphrina  pruni 
on  a  plum,  showing 
the  asci  developed 
without  any  cover- 
ing on  the  surface 
of  the  epidermis 
(X400).  Redrawn 
with  modifications 
from  Strasburger. 


YEASTS   (SACCHAROMYCES) 


377 


Certain  species^  give  desirable  flavors  to  some  kinds  of  cheese 
and  are  quite  usetul  in  tliis  connection. 

Naked-ascus  Fungi  (Protodiscales).  —  Tiiis  is  a  small  group  of 
parasites  which  attack  seed  plants.  They  produce  no  ascocarp 
and  the  asci  are  therefore  borne  exposed  {Fig.  328).  So  far  as 
known  they  have  no  sexual  reproduction.  They  are  regarded 
as  simple  Ascomycetes.  One  common  species  is  the  Exoascus 
deformans,  which  causes  the  disease  known  as  Peach  Curl.  The 
mycelium  develops  in  the  tissues  of  the  host  and  forms  on  the 
surface  asci  which  appear  as  gray  pow- 
dery films.  One  species  attacks  the 
young  ovaries  of  Plums,  causing  the 
malformation  known  as  "  Bladder 
Plums,"  and  one  species  causes  Witches' 
Brooms  on  some  of  our  deciduous  trees. 

Yeasts  (Saccharomyces).  —  The 
Yeasts  are  very  simple  Ascomycetes. 
In  most  Yeasts  the  hyphae  are  so  short 
and  simple  that  they  appear  as  single 
globular  cells  The  only  reason  for 
calling  them  Ascomycetes  is  that  under 
certain  conditions  the  cells  form  spores 
and  then  resemble  asci  {Fig.  329). 

On  account  of  their  ability  to  fer- 
ment sugars  and  produce  carbon  dioxide 
and  alcohol,  they  are  useful  in  making 
bread  and   in    making   alcohol,    wine, 

beer,  and  other  liquors  which  contain  alcohol.  When  placed  in 
dough  they  grow  and  work  rapidly,  and  the  carbon  dioxide  pro- 
duced causes  the  bread  to  rise.  There  are  many  kinds  of  Yeasts, 
and  each  kind  gives  a  different  flavor  to  the  fermented  product. 
For  this  reason  brewers  keep  pure  cultures  of  certain  kinds  of 
Yeasts,  which  give  the  liquor  the  desired  characteristics. 

Their  main  method  of  reproduction  is  by  the  rapid  division  of 
cells,  often  called  budding,  in  which  small  cells  are  apparently 
pinched  off  from  the  parent  cell.  The  cells  often  remain  in 
contact  for  some  time  after  being  budded  off,  forming  chains  of 
cells. 


Fig.  329.  — Bread  Yeast, 

Saccharomyces  cerevisiae.  a, 
single  plant  (X  600);  b,  a 
plant  in  the  process  of  bud- 
ding; c,  plant  which  has 
formed  spores;  d,  plants  re- 
maining in  contact  and 
forming  chains  as  they  are 
multiplied  by  budding. 


'  Cultural  Studies  of  Species  of   Penicillium. 
Animal  Industry,  U.  S.  Dept.  Agriculture,  1911. 


Bulletin   I4S,  Bureau  of 


378  THALLOPHYTES 

Other  Ascomycetes.  —  A  study  of  a  few  types  of  the  Ascomy- 
cetes  has  giveu  a  general  notion  of  their  habits  but  no  notion 
at  all  of  their  extensive  number.  However,  with  this  general 
acquaintance,  other  forms  can  be  easily  understood.  Some  other 
common  destructive  forms  are  the  Apple  and  Pear  Scab  ^  (Fig. 
330),  the  Bitter  Rot  of  Apples  2  (Fi^.  331),  Peach  Mildew,^  Black 


Fig.  330.  —  Apple  attacked  by  Scab,  Venturia  Pomi.     Photographed 
by  Whetzel. 

Rot  of  Grapes/  and  the  Wilt  disease  of  Cotton,  Watermelons, 
and  Cowpeas,^  etc. 

Summary  of  Ascomycetes.  —  The  Ascomycetes  have  no  water 
habits  and  their  chief  resemblance  to  the  Algae  is  in  the  character 
of  their  sex  organs  and  fruiting  bodies.     The  plant  body  ranges 

1  A  Contribution  to  Our  Knowledge  of  Apple  Scab.  Bulletin  96,  Mon- 
tana Agr.  Col.  Exp.  Sta.,  1914. 

-  Bitter  Rot  of  Apples.  Bulletin  U,  Bur.  PI.  Ind.,  U.  S.  Dept.  of  Agricul- 
ture, 1903. 

2  Peach  Mildew.     Bulletin  107,  Colorado  Agr.  Exp.  Sta.,  1906. 

^  The  Control  of  Black-Rot  of  Grape.  Bulletin  155,  Bur.  PI.  Ind.,  U.  S. 
Dept.  Agricultiu-e,  1909. 

^  Wilt  Disease  of  Cotton,  Watermelon,  and  Cowpea.  Bulletin  17,  Division 
of  Vegetable  Path.,  U.  S.  Dept.  Agriculture,  1899. 

Also  see  Spraying  Practice  for  Orchard  and  Garden.  Bulletin  127,  Iowa 
Agr.  Exp.  Sta.,  1912. 


LICHENS  379 

from  a  single  cell,  as  in  Yeast,  to  a  massive  mycelium  which  in 
some  cases  takes  no  definite  shape  while  in  others  it  forms  a 
definitely  shaped  fruiting  body.  In  parasitic  forms  the  mycelium 
sometimes  runs  through  the  tissues  of  the  hosts,  and  sometimes 
is  chiefly  superficial,  sending  only  haustoria  into  the  host. 


Fig.  331.  —  Apple  attacked  by  the  Bitter  Rot  Fungus,  Glomerella  rufu/naculans. 
After  Alwood. 


The  spores  are  of  two  kinds,  conidiospores  and  ascospores. 
The  conidiospores  are  borne  free  on  projecting  hyphae,  and  grow 
directly  into  hyphae  upon  germination.  The  ascospores,  the 
characteristic  spores  of  the  group,  are  borne  in  asci  which  are 
usually  produced  within  a  fruiting  body  or  ascocarp,  which  may 
be  an  open  structure  or  a  closed  one. 

In  controlling  the  disease-producing  forms  one  must  reckon 
with  conidiospores  and  ascospores. 


Lichens 

Lichens  are  very  common  structures  which  form  splotches  on 
stumps,  tree  trunks,  rocks,  old  boards,  etc.,  and  some  grow  upon 
the  ground.  Figure  332  shows  an  Apple  twig  covered  with 
Lichens.  They  may  appear  as  a  crust  covering  the  support ;  or 
they  may  have  flat  lobed  bodies  like  the  one  shown  in  Figure  333; 


380 


THALLOPHYTES 


or  they  may  have  slender  branching  bodies  Kke  the  one  shown 
in  Figure  334-  The  slender  branches  may  be  erect,  prostrate, 
^^^  or  hang  in  festoons  from  the 
?)  1^1^  branches  of  trees  or  other  sup- 
ports. 

A  Lichen,  although  regarded 
as  a  plant,  is  a  structure  formed 
by  the  association  of  a  Fungus 
and  an  Alga.  The  Fungus  in- 
volved is  in  nearly  all  cases  an  As- 
comycete,  and  the  Alga  involved 
is  nearly  always  a  unicellular 
form  of  the  Green  Algae  or  some 
form  of  the  Blue-green  Algae. 
The  Fungus  is  a  parasite  on  the 
Alga,  obtaining  food  from  the 
Alga.  The  hyphae  of  the  Fungus 
get  food  from  the  Alga  by  being 
in  close  contact,  and  since  the 
cells  of  the  Alga  are  rarely  pene- 
trated, the  Alga  apparently  is  not 
A  section  through  a  Lichen,  as  shown  in 


Fig.  332.  —  Lichens  on  an  Apple 
branch.  From  Bulletin  185,  Maine 
Agr.  Exp.  Sta. 


injured  in  most  cases. 
Figure  335,  shows  a 
meshwork  of  hyphae 
and  in  the  meshes  the 
cells  of  the  Alga  are 
held.  Usually  the  hy- 
phae are  more  closely 
interwoven  in  the  outer 
region,  thus  forming  a 
compact  cortical  region 
which  encloses  the 
looser  region  within 
where  the  cells  of  the 
Alga  are  usually  more 
abundant.  On  the 
under  surface   filamen- 


FiG.  333.  —  A  Lichen  with  a  flat  lobed  body 
growing  on  bark.  The  asci  are  produced  in 
the  small  cups.     X  h- 


tons  structures  are  developed  which  attach  the  plant  body  to  the 
substratum.  The  mycelium  of  the  Fungus  thus  constitutes  the 
framework  of  the  plant  body  or  thallus. 


LICHENS  381 

The  two  plants  of  this  association  arc  of  mutual  help.  The 
sponge  structure  formed  by  the  Fungus  holds  water  for  the  Alga, 
while  the  Alga  makes  carbohydrates,  some  of  which  can  be  used 
by  the  Fungus.  As  a  result  of  this  mutual  help;  the  Lichen  can 
live  on  dry  barren  rocks  where  other  plants  cannot  exist.     Neither 


Fig.    335.  —  A   much   en- 
larged    section     through     a 
Fig.  334.  —  A   much  branched  Lichen,  showing    the    fungal 

Lichen  hanging  from  the  branch  liyphae  and  the  globular  cells 

of  a  tree.  of  the  Alga. 

the  Alga  nor  the  Fungus  could  grow  in  such  places  alone,  for  the 
Alga  would  lack  moisture  and  the  Fungus  would  lack  food. 
Being  so  little  dependent  upon  their  support  for  moisture  and 
food,  the  Lichens  are  the  pioneers  on  bare  and  exposed  surfaces. 
They  hasten  the  disintegration  of  rock  and  start  soil  formation. 
The  materials  of  their  dead  bodies  added  to  the  disintegrate  rock 
form  a  soil  for  other  plants. 

Lichens  multiply  vegetatively  by  small  scale-like  portions, 
called  soredia,  which  separate  from  the  main  plant  body.  Soredia 
are  small  masses  of  hyphae  in  which  some  algal  cells  are  en- 
tangled and  are  capable  of  growing  directly  into  Lichens. 

The  fungal  member  of  Lichens  usuall}^  reproduces  by  asco- 
spores  and  the  algal  member  by  cell  division.  The  asci  occur  in 
ascocarps  which  appear  as  small  cups  or  disk-like  bodies  on  the 
surface  of  the  plant  body  (Fig.  336).  The  sex  organs  are  quite 
suggestive  of  the  Red  Algae.  The  antheridia  occur  on  branching 
hyphae  and  are  very  small  cells  which  break  off  and  function  as 
sperms.  After  fertilization,  sterile  hyphae  grow  up  from  below 
the  ascogonium  and  form  the  wall  of  the  ascocarp  which  finally 


382 


THALLOPIIYTES 


breaks  through  and  appears  on  the  surface  of  the  phint  body  as 

a  cup  or  disk. 

Besides  being  the  pioneer  plants  on  rocks  and  other  places 

where  they  form  soil  and  thus  make  it  possible  for  higher  plants 

to   get   a   start,  they  are   also   of   some   economic    importance 

in  other  ways.  In  northern  re- 
gions the  Lichen  known  as  Rein- 
deer Moss  is  an  important  food 
for  animals.  Some  forms  are 
used  as  food  by  man.  Although 
not  parasites,  they  sometimes  are 
harmful  to  plants  upon  which 
they  grow.  When  growing  on 
the  twigs  of  fruit  trees,  they  pre- 
vent the  bark  from  functioning 
properly  and  also  furnish  a  shelter 
for  various  kinds  of  destructive 
insects. 

Basidiomycetes 

General  Description.  —  This  is 
the  group  of  P'ungi  to  which 
Toadstools,  Mushrooms,  Puff- 
balls,  Rusts,  and  Smuts  belong. 
The  group  scarcely  needs  an  intro- 
duction, because  such  conspicu- 
ous forms,  as  Toadstools,  Mush- 
rooms, and  Puff  balls  are  famiUar. 
to  everybody.  In  number  of  forms  this  group  is  next  to  the 
Ascomycetes.  Their  characteristic  spore-bearing  structure  is  the 
hasidium,  which  is  the  enlarged  end  of  a  hypha  with  usually  four 
slender  branches  upon  which  spores  are  borne,  one  spore  being 
borne  on  the  end  of  each  branch.  Just  as  the  spores  borne  in  an 
ascus  are  called  ascospores  and  are  the  characteristic  spores  of 
the  Ascomycetes,  so  those  borne  on  a  basidium  are  called  basidi- 
ospores  and  are  the  characteristic  spores  of  the  Basidiomycetes. 
The  mycelium  of  many  is  saprophytic,  living  in  decaying  wood, 
rotten  manure,  and  other  kinds  of  organic  matter.  In  others, 
such  as  the  Rusts,  Smuts,  and  other  forms,  the  mycelium  is 
parasitic,  living  upon  the  tissues  of  the  grains  and  other  higher 


Fig.  336.  —  Reproduction  in 
Lichens  by  ascospores.  Above, 
vertical  section  through  a  cup 
(apothecium),  showing  asci  and 
paraphyses;  below,  asci  and  pa- 
raphyses  shown  more  enlarged. 
Redrawn  from  Schneider. 


GENERAL   DESCRIPTION  383 

plants.  Even  the  sapropliytic  forms  causo  some  undesirable 
destruction.  They  often  start  in  the  wounds  of  fruit  trees,  shade 
trees,  and  forest  trees,  and  the  action  of  their  mycelia  hastens 
decay  and  may  lead  to  the  destruction  of  the  tree. 

In  many  forms  the  mycelium,  after  it  is  well  established  in  the 
region  of  food  supply,  produces  on  the  surface  of  the  substratum 
some  kind  of  a  body  in  which  the  spores  are  borne.  It  will  be 
recalled  that  this  is  the  habit  of  the  Morel.  This  body,  since  it 
bears  the  spore,  is  called  a  sporophore  which  really  means  a 
"spore-bearing  body."  It  is  a  term  commonly  applied  to  a 
spore-bearing  hyphae  or  to  any  portion  or  all  of  the  plant  body 
which  has  to  do  with  bearing  spores.  Thus  the  wrinkled  top  and 
stalk  bearing  it  constitutes  the  sporophore  in  the  Morel.  In  the 
Toadstools  and  Mushrooms,  the  sporophore  is  often  umbrella- 
shaped.  In  some  forms  which  grow  on  the  sides  of  trees  and 
stumps,  the  sporophore  resembles  a  small  shelf  projecting  from 
the  support,  and  in  this  case  the  sporophore  is  often  hard.  In 
PufTballs  the  sporophore  is  more  or  less  globular.  Sporophores 
are  extremely  variable  in  both  shape  and  texture,  and  are  the 
structures  by  which  those  Fungi  which  have  them  are  classified. 
The  sporophore  is  the  part  of  the  Fungus  that  attracts  attention. 
It  is  the  portion  that  is  eaten  and  called  a  Mushroom.  The 
portion  of  the  mycelium  which  traverses  the  substratum  is  usually 
hidden,  and  its  presence  is  not  known  until  the  sporophore 
appears. 

Many  of  the  parasitic  Basidiomycetes,  Hke  the  Smuts  and 
Rusts,  have  no  conspicuous  sporophores,  and  the  presence  of  the 
mycelium  is  indicated  only  by  the  occurrence  of  unusual  struc- 
tures on  the  surface  of  the  host  plant.  In  case  of  Smut  the  pres- 
ence of  the  disease  is  indicated  by  the  appearance  of  Smut  balls, 
and  in  Rusts,  by  the  red  or  black  blisters  occurring  on  the  leaves 
and  stem  of  the  host. 

Although  the  basidiospores  are  the  characteristic  spores  of  the 
group,  a  number  of  other  kinds  of  spores  occur,  which  in  some 
cases  are  more  important  in  reproduction  than  the  basidiospores. 
Sexual  reproduction  has  been  entirely  lost  by  many  of  the  group, 
and  in  those  where  it  is  retained  the  fusion  is  between  hyphae, 
there  being  no  sex  organs  formed.  There  are  no  oospores  or 
zygospores  to  be  considered  in  this  group. 

The  Basidiomycetes,  of  which  there  are  14,000  or  more  species, 


384 


THALLOPIIYTES 


are  divided  into  a  nunilxn-  of  orders.  The  most  familiar  orders 
are  those  represented  by  the  Toadstools  and  Mushrooms  {Hy- 
menomycetes),  Puffballs  (Gasteromycetes),  Smuts  {Ustilaginales), 
and  Rusts  (Uredlitdlcs). 

Toadstools  and  Mushrooms  (Hymenomycetes).  —  This  is  the 
most  familiar  order  to  most  people,  because  it  includes  so  many 
forms  like  the  Toadstools  and  Mushrooms,  which  have  conspicu- 
ous sporophores.  In  addition  to  the  Toadstools  and  Mushrooms, 
the  order  contains  some  other  rather  familiar  kinds  of  Fungi. 
The  Fungi  of  this  order  are  chiefly 
saprophytes,  living  on  decaying  wood, 
leaf  mold,  rich  humus,  and  manure. 
Often  the  organic  matter  upon  which 
they  are  living  is  not  visible  and  they 
seem  to  be  growing  right  out  of  the 
soil.  As  the  name  of  the  order  sug- 
gests, they  have  a  hymenium,  and 
the  hymenium,  which  consists  of 
basidia  commonly  intermingled  with 
sterile  hyphae,  is  borne  exposed. 
Usually  the  hymenium  is  on  the 
under  side  of  the  sporophore  where  it 
is  protected  from  rain. 

Those  of  the  order  having  umbrella- 
shaped  sporophores  are  popularly 
called  Toadstools  and  when  edible 
they  are  popularly  called  Mushrooms. 
The  term  Mushroom,  however,  is 
often  applied  to  Morels  and  all  kinds 
of  Fungi  that  are  edible.  There  are 
several  hundred  species  of  edible  Fungi  in  the  United  States 
and  more  than  one  hundred  of  them  are  of  the  Toadstool  type. 
Some  of  the  Toadstools  are  deadly  poisonous,  as  the  one  shown 
in  Figure  337,  and  many  that  are  not  poisonous  are  tough, 
fibrous,  or  ill-tasting  and  hence  not  edible.  Between  edible  and 
non-edible  Fungi  there  are  no  botanical  distinctions  or  guides. 
By  experience  people  have  learned  that  some  species  are  edible 
and  some  non-edible,  and  many  sad  accidents  have  occurred  as  a 
result  of  not  being  able  to  distinguish  the  poisonous  from  the 
edible  ones. 


Fig.  337.  —  A  i)oisonous 
Toadstool,  Amanita  bulbosa. 
X  I. 


TOADSTOOLS  AND   MUSHROOMS   (HYMENOMVCETES)      385 

The  order  is  divided  scientifically  into  a  number  of  sub- 
groups according  to  the  method  of  exposing  the  hymenium.  In 
the  largest  and  most  important  group  of  Hymenomycetes,  the 
hymenium  covers  the  surface  of  thin  radiating  plates  called  gills. 
These  Fungi  are  known  as  the  Agarics  or  Gill  Fungi.  To  the 
Gill  Fungi  belong  most  Toadstools  and  the  Field  Mushroom 
{Agaricus  camjjestris)  which  is  extensively  cultivated  for  market. 


Fig.  338.  — •  Stages  in  the  development  of  the  Mushroom,  Agaricus  cam- 
pestris.  I,  ground  Hne;  m,  underground  portion  of  mycelium;  s,  stipe; 
p,  pileus;  g,  gills;  a,  annulus.      X  2- 

On  account  of  their  structural  complexity  the  Agarics  are  re- 
garded as  highly  developed  Fungi.  They  develop  as  shown  in 
Figure  338. 

Before  developing  the  sporophore,  the  mycelium  becomes  well 
established  in  decaying  organic  matter  and  this  may  require 
considerable  time.  In  the  development  of  a  sporophore,  there 
first  appears  on  the  surface  of  the  substratum  a  small  spherical 
body  called  a  button  which  has  a  skin-like  covering  witliin  which 
the  sporophore  is  forming.     This  body  elongates  very  rapidly  if 


386 


THALLOPHYTES 


the  weather  is  warm  ami  moist  aiul  sometimes  the  sporophor.' 
attains  full  size  in  a  few  hours.  The  elongating  sporophore 
finally  breaks  through  the  covering  of  the  button,  spreads  out  its 
umbrella-like  top,  and  the  characteristic  sporophore  appears  with 
lemnants  of  the  torn  skin-like  covering  remaining  attached. 

When  mature  the  sporophore  consists  of  a  stalk,  called  stipe, 
and  the  expanded  uml)rella-like  top,  called  pileus.     On  the  under 


Fig.  339.  —  Reproductive  structures  of  the  Mushroom,  Agaricus  cam- 
pcstri!i.  A,  the  Mushroom  with  a  portion  of  its  pileus  cut  away  to  show 
the  gills,  g,  gills;  s,  stipe;  a,  amiulus.  B,  section  through  a  gill,  highly 
magnified  to  show  the  basidia  (b)  and  the  basidiospores  (r) .  Redrawn  from 
Leavitt. 


side  of  the  pileus  arc  the  thin  radiating  plates  or  gills  bearing  the 
hymenium  in  which  occur  the  basidia  as  shown  in  Figure  339.  A 
fragment  of  the  skin-like  covering  of  the  button  stage  commonly 
remains  attached  to  the  stipe,  forming  the  annulus  and  in  some 
forms,  as  shown  in  Figure  337,  a  portion  of  the  covering  remains 
as  a  cup  at  the  base  of  the  stipe,  forming  the  volva.  Other  frag- 
ments of  the  covering  often  remain  as  flecks  on  the  outer  surface 
of  the  pileus.     When  the  spores  are  mature,  they  fall  from  the 


TOADSTOOLS  AND   MUSHROOMS   (HYMKNOMYCETES)      387 

basidia  and  may  reach  the  ground  directly  beneath  or  be  carried 
away  by  the  wind.  When  favorably  situated,  the  spores  grow 
new  mycelia,  thus  com- 
pleting the  round  of  life. 
The  basidiospore  is  the 
only  spore  formed  and  no 
sexuality  has  been  dis- 
covered. 

Small  brick-like  masses 
of  organic  matter,  usually 
consisting  of  manure  and 
containing  mycelial 
threads  of  the  Mushroom 
in  a  dormant  state,  are 
sold  on  the  market,  and 
used  in  starting  Mush- 
room beds,  the  mycelial 
threads  contained  consti- 

tuting  the  so-called  Mush-  ^^  ^^^  _  ^j^^  j,^^j^  g^j^^^^^  ^ 

room  spawn.  polyporus  Fungus.     X  i. 

In  another  rather  com- 
mon family  ( Polyporaceae)  of  the  Hymenomycetes,  the  hymenium 


Fig.  341.  —  A  Ilydnum,  a 
Fungus  in  wliich  the  hymenium 
is  borne  on  tooth-like  projec- 
tions.    X  h 


Fig.  342.  — a  Basidio- 
mycete,  Clavaria,  with  n 
much  branched  sporo- 
phore.     X  5. 


lines  tubes  with  pore-like  openings.  These  are  known  as  the  Pore 
Fungi,  and  to  this  family  belong  some  Toadstools,  some  of  which 
are  edible  (^Fig.  S40),  and  the  Bracket  Fungi^  which  form  shelf- 


388  THALLOPHYTES 

like  sporophores  on  the  sides  of  trees  and  stumps.  In  the  family 
to  which  the  Hydnums  belong  the  hymenium  is  borne  on  tooth- 
hke  projections  {Fig.  341)-  In  another  family  the  sporophore 
is  much  branched  and  the  hymenium  covers  the  surface  of  the 
branches  {Fig.  342).  As  to  the  texture  of  the  sporophore,  that 
varies  widely  in  the  different  families.  In  some  families  it  is 
gelatinous  and  without  definite  shape.  It  is  fleshy  in  the  Toad- 
stools and  Mushrooms  and  in  some  of  the  Bracket  Fungi  it  be- 
comes as  hard  and  persistent  as  wood. 


Fig.  343.  —  Some  of  the  roots  and  the  lower  portion  of  the  trunk  of 
Apple  tree  which  has  been  killed  by  the  Toadstools. 


an 


Destructive  Toadstools  and  Bracket  Fungi.  —  Some  Toad- 
stools attack  the  roots  of  trees  and  cause  the  disease  called  Root 
Rot.  This  disease  occurs  on  a  number  of  fruit  trees,  such  as  the 
Apple,  Plum,  Cherry,  and  Peach,  and  on  many  shrubs  and  forest 
trees.  In  Figure  343  is  shown  some  Toadstools  which  have 
destroyed  an  Apple  tree.  The  Toadstools  usually  cause  the  death 
of  the  roots,  and  this  results  in  killing  the  tree.  The  mycelia  of 
the  Toadstools  probably  enter  the  roots  through  wounds. 


rUFFBALLS   AND   RELATED   FORMS    (GASTEROMYCETES) 


389 


In  Figure  344  is  a  Bracket  Fungus  whic^h  causes  a  disease 
known  as  White  Heart  Rut.  This  disease  occurs  on  fruit  trees 
and  many  forest  trees.  The  spore  enters  tiirough  a  wound  and 
starts  the  myceUum  which  penetrates  and  transforms  the  heart 
wood  into  a  white  pulpy  mass.  In  Figure  345  is  shown  another 
Bracket  Fungus  which  attacks  trees  in  a  similar  way  and  causes 
the  wood  to  rot  and  become  reddish  brown  or  black.  It  produces 
the  Red  Heart  Rot.  There  are  many  other  destructive  forms 
which   concern  the   forester  and  horticulturist.     They  start  in 


Fig.  344.  —  One  of  the  Bracket  Fungi,  Fojnes  igniarim,  living  on  the  trunk 
of  a  Hving  Aspen.  It  attacks  various  trees,  destroying  the  wood  and  causing 
much  damage.  From  Bulletin  189,  Bureau  of  Plant  Industry,  U.  S.  Dept. 
of  Agriculture. 

wounds  where  there  is  some  decaying  matter,  and  in  pruning  it 
is  necessary  to  guard  against  the  entrance  of  these  Fungi. 

Puffballs  and  Related  Forms  (Gasteromycetes).  —  On  account 
of  the  complexity  of  their  sporophores,  the  Gasteromycetes  are 
considered  the  highest  of  all  Fungi.  They  are  saprophytes, 
growing  on  decaying  wood,  leaf  mold,  rich  humus,  and  manure. 
They  require  about  the  same  conditions  for  growth  as  do  the  Toad- 
stools and  Mushrooms  and  are  often  found  growing  with  them. 
There  are  about  700  species,  many  of  which  are  edible.  The 
sporophore  of  these  Fungi  is  usually  more  or  less  glol)ular  in  form 
and  the  hymenium  is  enclosed. 


390 


THALLOPHYTES 


The  most  common  and  familiar  members  of  the  order  are  the 
Puffballs,  common  in  the  woods  and  fields,  and  so  named  because 
when  pressed  upon  the  spores  puff  out  in  cloud-like  masses 
(Fig.  346).  Some  of  the  Puffballs  are  a  foot  or  more  in  diameter 
when  mature  and  most  of  them  are  edil)le.     The  sporophore 


Fig.  345.  —  A  Polyporus  Fungus,  Fulyporus  sulfureus,  on  the  RcmI  Oak. 
It  causes  the  Red  Heart  Rot  of  trees.  Photo  by  Dr.  W.  A.  Murrill,  N.  Y. 
Botanical  Garden. 


develops  from  a  subterranean  mycelium,  and  is  differentiated 
into  an  outer  region  which  constitutes  a  two-layered  skin-like 
covering  i'peridium)  and  an  interior  chambered  region  (gleba)  in 
which  the  basidia  intermingled  with  sterile  hyphae  occur.  Spores 
are  produced  in  immcmse  numbers.  A  Puffball  of  ordinary  size 
produces  many  millions  of  spores.  The  spores  are  dark  in  color 
due  to  their  heavy  walls.  They  escape  from  the  sporophore 
through  pore-like  or  slit-like  openings  in  the  peridium. 


PUPTBALL8   AND   RELATED   FORMS    (GASTEROMYCETES)      391 


A  very  interesting  Puffball  is  the  Earthstar  (Geader)  shown  in 
Figure  347.  In  this  form  the  outer  hiyer  of  the  peridiuni  sjolits 
into  regular  segments  and  these  segments  are  hygroscopic. 
When  the  segments  are  wet  they  bend  l^ack  and  downwartl  and 
in  this  way  the  outer  layer 
of  the  peridium  spreads  out 
like  a  star.  The  inner  layer 
of  the  peridium  opens  by  an 
apical  pore  and  allows  the 
spores  to  escape  as  in  other 
Puffballs. 

The  Bird's  Nest  Fungi 
(Fig.  348),  which  are  close 
relatives  of  the  Puffballs, 
show  another  interesting  fea- 
ture. They  are  small,  usu- 
ally less  than  a  centimeter 
in  height  and  width.  They 
develop  on  twigs  and  sticks 
as  well  as  on  organic  matter 

that  is  quite  well  decayed.    One  often  finds  them  growing  on  the 
benches  in  greenhouses.     The  chambers  of  the  gleba  become 


Fig.  346.  —  Puffballs,  Lyco] 
Three  have  opened  at  the  top,  thus 
allowing  the  spores  produced  in  the  in- 
terior to  escape.     X  i- 


Fig.  347.  —  An  Earthstar,  Gtaster.     About  natural  size. 


enclosed  in  walls  and  separate.  After  the  peridium  opens,  the 
sporophore  is  cup-shaped  and,  with  the  egg-like  chambers  of  the 
gleba  exposed,  resembles  a  bird's  nest  full  of  eggs. 

The  Stink  Horn  (Fig.  349),  noted  for  its  intolerable  odor,  is 
another  Fungus  of  this  order.     Its  mycelium  feeds  on  decaying 


392 


THALLOrilYTES 


Fig.  348. —  a  Bird's 
Nest  Fungus,  Nidularia. 
About  natural  size. 


organic  mattor  in  the  ground.  The  si)orophoro  is  at  first  globose, 
but  the  glel)a  soon  breaks  out  of  the  peridiuni  and  is  elevated  to 
some  distance  above  ground  by  an  elongating  stalk.  The  spore 
masses  are  slimy  and  have  the  odor  of 
carrion.  Certain  insects  which  dissemi- 
nate the  spores  are  attracted  by  the 
odor. 

Smuts  (Ustilaginales) .  —  The  Smuts 
are  parasitic  Basidiomycetes.  In  some 
Smuts,  the  mycelium,  although  evident 
only  in  local  areas,  traverses  widely 
through  the  host,  while  in  others  only 
local  areas  of  the  host  are  attacked. 
No  sporophores,  such  as  characterize 
the  Toadstools  and  Puffballs,  occur  in  the  Smuts.  There  are 
more  than  2000  species  of  Smuts.  They  attack  chiefly  plants 
of  the  Grass  family  and  espe- 
cially the  cereals,  the  grains  of 
which  they  commonly  displace 
with  powdery  l^lack  masses  of 
spores.  The  financial  loss  due 
to  Oat  Smut  alone  has  been 
estimated  to  be  $10,000,000 
annually  in  the  United  States. 
In  addition  to  the  loss  due  to 
the  destruction  of  the  cereal 
crops  and  the  lowering  of  their 
market  price,  there  is  consider- 
able loss  due  to  Smut  explosions 
in  thrashing  machines.  During 
the  summer  of  1914,  300  thrash- 
ing machines  were  blown  up  or 
Ijurned  in  the  Pacific  Northwest 
by  Smut  explosions.  Smut  dust 
is  highly  combustible  when  dry, 
and  is  probably  ignited  by  static 
electricity  in  the  cylinder  of  the 

thrashing  machine.  The  Smuts  are  particularly  destructive 
to  Oats,  Wheat,  Rye,  and  Barley.  Corn  Smut  is  exceedingly 
common  but  less  destructive. 


Fig.  349.  —  Stink  Horn  Fungus, 
Phallus  i77ipudicus.  At  the  right, 
vertical  section  of  the  Fungus  in 
early  stage  of  development,  showing 
the  gleba  enclosed  by  the  peridium. 
At  the  left,  mature  stage,  showing 
the  gleba  elevated  much  above  the 
peridium.      X  3. 


THE  SMUT  OF  OATS 


393 


The  Smut  of  Oats.^  —  The  Smut  of  the  Oats  is  probably  the 
most  common  and  destructive  one  of  the  Smut  group.  The 
mycelium  of  the  Oat  Smut  gets  started  in  the  tissues  of  the  Oat 
plant  when  the  latter  is  in  the  seedling  stage,  and  at  flowering 


Fig.  350.  —  Loose  Smut  of  Oats.     Left,  normal  head;     right,  head 
destroyed  by  Smut.     After  Bulletin  112,  Miimesota  Agr.  Exp.  Sta. 

time  it  masses  in  the  ovaries,  which  become  swollen  and  finally 
destroyed   and   replaced   by   masses   of   spores    {Fig.    350).     A 

'  The  following  references  will  be  found  helpful  in  understanding  the 
smuts  and  methods  of  combatting  them. 

The  Grain  Smuts.     Farmers'  Bulletin  75,  U.  S.  Dept.  of  Agriculture,  1898. 

Corn  Smut.     Annual  Report  12,  Indiana  Agr.  Exp.  Sta.,  1900. 

The  prevention  of  Stinking  Smut  of  Wheat  and  Loose  Smut  of  Oats.     Farm- 
ers' Bulletin  250,  U.  S.  Dept.  Agriculture,  1906. 

The  Smuts  of  Grain  plants.     Bulletin  122,  Miimesota  Agr.  Exp.  Sta.,  1911. 

The  Smuts  of  ^\^^[cat,  Oats,  Barley,  and  Corn.     Farmers'  Bulletin  507,  U.  S. 
Dept.  Agriculture,  1912. 

Bunt  or  Stinking  Smut  of  \\lioat.     Bulletin  126,  Washington  Agr.  Exp.  Sta., 
1915. 


394 


THALLOPHYTES 


study  of  the  forniution  of  these  spores  shows  that  they  are  not 
basidiospores,  for  they  are  not  formed  on  basidia.  The  hyphae 
m  the  smut  ball  simply  divide  into  cells  which  separate  and 
become  spores.  These  spores  are  the  so-called  brand  spores,  the 
whole  mass  of  them  forming  the  so-called  Smut.  The  spores  are 
very  heavy-walled  and  appear  black  in  mass.  This  kind  of  a 
heavy-walled  spore,  which  is  simply  a  transformed  vegetative 
cell  of  the  mycelium,  is  called  a  chlamydospore,  a  name  referring 
to  the  heavy  protective  wall.  The  spore  masses  break  up  when 
mature  and  the  spores  are  shed.  In  han- 
dling the  grain,  especially  in  thrashing,  the 
spores  escape  in  dust-like  fogs.  The  spores 
pass  the  winter  on  the  ground,  straw,  grain, 
or  wherever  they  happen  to  fall.  Many 
of  the  spores  lodge  on  the  Oat  grain,  fall- 
ing down  between  the  lemma  and  palea 
which  enclose  the  Oat  kernel.  The  follow- 
ing spring  the  chlamydospores  germinate, 
each  producing  a  small  hypha  called  a  pro- 
mycelium,  on  which  the  basidiospores  are  pro- 
duced. The  basidiospores  are  produced  on 
the  end  and  sides  of  the  promycelium  as 
shown  in  Figure  351.  Their  number  is  in- 
definite and  they  often  multiply  by  budding 
after  the  manner  of  the  Yeasts.  They  are 
(luite  commonly  called  conidia  and  often 
sporidia,  although  they  are  comparable  to  the 
basidiospores  of  the  Toadstools  and  Puff- 
balls.  It  is  on  account  of  the  occurrence  of 
the  promycelium,  which  is  regarded  as  a 
basidium,  that  the  Smuts  are  classed  as  Basidiomycetes.  Once 
in  contact  with  a  young  Oat  plant,  the  basidiospores  produce 
hyphae,  known  as  infection  hyphae,  which  penetrate  the  young 
plant  and  start  the  development  of  a  mycelium. 

It  has  been  found  that  most  of  the  infection  in  Oat  Smut 
results  from  the  chlamydospores  which  are  lodged  on  the  grain, 
and  that  by  soaking  seed  Oats  in  hot  water  (132°  to  133°  F.)  for 
ten  to  fifteen  minutes  or  in  water  containing  about  1  pint  of 
40  per  cent  formalin  to  45  gallons  of  water,  the  spores  can  be 
killed  and  much  loss  to  the  Oat  crop  prevented. 


Fig.  351.  —  Ger- 
mination of  Chlamy- 
dospores. At  the 
left,  a  spore,  and  at 
the  right,  a  spore 
which  has  germinated 
and  produced  a  pro- 
mycelimn  bearing 
basidiospores  (c).  X 
about  300. 


CORN  SMUT 


395 


The  Smut  of  Oats,  Stinking  Smut  of  Wheat,  and  Covered  Smut 
of  Barley  are  very  similar  in  habit  and  require  similar  treatment. 
Sometimes,  as  in  case  of  the  Stinking  Smut  of  Wheat,  the  infec- 
tion of  the  seedling  may  be  due  to  spores  lodged  in  the  soil  as 
well  as  to  spores  adhering  to  the  kernel. 

Loose  Smuts  of  Wheat  and  Barley.  —  The  Loose  Smuts  of 
Wheat  and  Barley  mature  and  shed  their  chlamydospores  when 
the  grain  is  in  flower.  These 
spores  are  borne  away  by  the 
wind  and  when  falling  on  the 
flowers  of  their  respective 
hosts,  grow  hyphae  into  the 
young  kernel.  The  kernel 
continues  its  development, 
but  when  mature  it  has  con- 
cealed within  a  tiny  Smut 
plant,  which  is  able,  when  the 
kernel  is  planted,  to  resume 
its  growth  and  develop  in  ttie 
grain  plant.  Much  of  the 
damage  from  these  Smuts 
can  be  avoided  by  seed  selec- 
tion. Treatments  for  these 
Smuts  must  aim  at  killing 
the  tiny  Smut  plants  con- 
cealed in  the  seed  grain. 
Soaking  the  seed  in  cold 
water  five  hours  and  then  in 
water  130°  F.  for  ten  minutes 
is  recommended. 

Corn  Smut.  —  Corn  Smut 
is  the  most  conspicuous  of 
the  Smut  group.     It  attacks 

all  tender  regions  of  the  Corn  plant  but  does  most  damage  to  the 
flowers  which  become  much  enlarged  and  transformed  into  Smut 
balls.  Tumor-like  developments  of  the  Fungus  occur  also  on  the 
leaves  and  stem  as  well  as  on  the  ear  and  tassel.  In  Figure  352 
is  shown  an  ear  in  which  the  kernels  are  rei^laced  by  the  tumor- 
like masses  of  the  Fungus.  These  Smut  Ijodies  have  a  thin, 
grayish,  hyphal  covering,  and  within  the  chlamydospores  are  pro- 


FiG.  352.  —  Ear  of  Corn  with  kernels 
destroyed  and  replaced  by  masses  of 
Smut.  From  Farmers'  Bulletin  507,  U.  S. 
Dept.  of  Afirirulture. 


396  THALLOPHYTES 

duced  by  the  division  of  hyphac  as  described  in  Oat  Smut. 
When  the  spores  are  mature,  the  skin-like  covering  breaks,  thus 
allowing  the  spores  to  be  scattered.  Some  spores  pass  the  winter 
on  the  old  stalks.  Others  pass  the  winter  on  the  ground  or  wher- 
ever they  happen  to  fall.  In  the  spring  the  chlamydospores  ger- 
minate and  produce  the  promycelia  and  basidiospores.  The 
basidiospores  are  blown  to  the  Corn  and  are  able  to  grow  hyphae 
into  the  tender  regions  of  the  plant  and  start  the  disease.  Treat- 
ment of  the  seed  Corn  is,  therefore,  of  little  value  in  combatting 
Corn  Smut.     In  what  way  can  Corn  Smut  be  controlled? 

Rusts  (Uredinales).^  —  Like  the  Smuts,  the  Rusts  are  internal 
parasites  and  only  their  spore  masses  are  visible  externally. 
They  are  so  named  on  account  of  the  red  color  of  their  spore 
masses.  There  are  about  2000  species  of  Rusts  and  they  attack 
nearly  all  kinds  of  plants  but  more  especially  members  of  the 
Grass  family.  Although  regarded  as  degraded  parasites,  they 
are  more  complex  than  the  Smuts,  for  they  have  more  kinds  of 
spores  and  many  of  them  have  alternating  stages  upon  different 
hosts.  For  example,  it  is  well  known  that  Wheat  Rust  and  the 
Common  Barberry  bush  (Berheris  vulgaris)  are  associated.  They 
are  associated  because  the  Wheat  Rust  lives  one  stage  of  its  life 
cycle  on  the  Wheat  and  the  other  on  the  Barberry.  Each  kind 
of  Rust  lives  on  only  certain  hosts  and  the  alternating  hosts  are 
plants  very  different  in  kind,  as  those  of  the  Wheat  Rust 
illustrate. 

Rusts,  although  directly  affecting  only  limited  areas  of  tissue 
around  the  places  of  attack,  commonly  attack  the  host  in  so 
many  places  that  they  weaken  the  host  and  thereby  prevent  grain 
plants  from  yielding  normally.  The  financial  loss  to  the  farmer 
due  to  Rusts  is  considerably  more  than  that  caused  by  Smuts. 
Some  years  the  loss  in  the  United  States  due  to  the  Black  Rust 
exceeds  $15,000,000.  The  Black  Rust  of  which  six  forms  are 
distinguished  is  the  most  important  one  of  the  Rusts. 

Black  Rust  of  Grain  (Puccinia  graminis).  —  The  Black  Rust, 
sometimes  called  Red  Rust,  is  a  dreaded  pest  on  Wheat,  Oats, 

'  Investigations  of  Rusts.  Bulletin  65,  Bureau  of  Plant  Industry,  U.  S. 
Dept.  Agriculture,  1904. 

Lessons  from  the  Grain  Rust  Epidemic  of  1904.  Farmers'  Bulletin  219, 
U.  S.  Dept.  Agriculture,  1905. 

Rust  of  Cereals.     Bulletin  109,  South  Dakota  Agr.  Exp.  Sta.,  1908. 


BLACK  RUST  OF   GRAIN    (PUCCINIA   GRAMINIS)        397 


Rye,  and  Barley,  and  occurs  on  other  Grasses.  The  presence  of 
the  mycehum  in  the  host  is  first  known  through  the  appearance  of 
reddish  spots  or  lines  on 
the  stems  and  leaves  in 
late  spring  or  early  sum- 
mer. The  reddish  spots 
or  lines  are  regions  of 
spore  production.  They 
are  pustules  or  blister- 
like  structures  caused  by 
masses  of  spore-bearing 
hyphae  which  push  up 
the  epidermis  until  it  is 
finally  ruptured  {Fig. 
353).  The  reddish  color 
of  the  pustules  is  due  to 
the  reddish  color  of  the 
spores.  These  spores  ar(^ 
known  as  the  "  summer 
spores "  or  uredosporefi. 
The  uredospores,  which 
are  produced  in  great 
numbers,  are  scattered  by  the 


Fig.  353.^  Wheat  Rust  as  it  appears  on 
Wheat.  Left,  portion  of  a  Wheat  plant, 
showing  the  pustules  on  the  stem  and  leaf; 
right,  a  much  enlarged  section  through  a  pus- 
tule, showing  the  summer  spores  (X  200). 


wind,  thus  reaching  other  host 
plants  into  which  they  grow 
hyphae  and  thereby  infect. 
They  are  chiefly  responsible 
for  the  rapid  spread  of  the 
disease  during  summer. 

Later  in  the  summer,  when 
the  grain  is  ripening  and  the 
food  for  the  Fungus  becomes 
scarce,  the  same  mycelia  pro- 
duce heavy-walled,  two-celled 
spores,  known  as  winter  spores 
or  teleutospores  {Fig.  354)- 
These  spores  are  dark  in  color, 
giving  the  pustules  a  dark  ap- 
pearance —  whence  the  name 
Black  Rust.  They  pass  the  winter  on  the  straw,  ground,  or  wher- 
ever they  happen  to  fall.     The  following  spring,  each  cell  of  the 


Fig.  354.  —  A  section  through  a  pus- 
tule in  late  summer,  showing  the  winter 
spores  or  teleutospores.     X  about  200. 


398 


THALLOPHYTES 


tcleutospore  produces  a  promycelium  bearing  the  basidiospores, 
often  called  sporidia,  as  shown  in  Figure  355.  Thus  the  tcleuto- 
spore occupies  the  same  position  in  the  life  history  of  Rusts  as  the 
brand  spore  occupies  in  the  life  history  of  Smuts.  The  basidio 
spores  are  scattered  by  the  wind,  and  in  regions  where  Barberry 
bushes  grow,  they  come  in  contact  with  the  leaves  of  the  Barberry 
where  they  grow  and  produce  mycelia  in  the  leaf  tissues. 
Upon  the  Barberry,  the  mycelia  produce  on 
the  under  surface  of  the  leaf  small  cups  called 
aecidia  in  which  spores  are  borne  in  chains 
as  shown  in  Figure  356.  These  spores  are 
called  aecidiospores  or  cup  spores.  The 
aecidiospores,  which  are  shed  in  the  spring 
or  early  summer,  are  disseminated  by  the 
wind  and  start  the  disease  on  the  grains  or 
other  Grasses,  thus  completing  the  life  cycle 
as  it  is  shown  in  Figure  357. 

In  connection  with  the  development  of  the 
aecidiospores  there  occur  on  the  upper  sur- 
face of  the  Barberry  leaf  very  small  flask- 
shaped  cups  called  spermagonia,  in  which  are 
produced  very  small  spores  called  spermatia 
or  pycniospores.  The  spermatia  have  no 
function  and  the  spermagonia  and  aecidia  are 
supposed  to  represent  the  remnants  of  a  sexual 
apparatus  which  has  become  functionless. 

Thus  four  kinds  of  spores  are  involved  in 
the  complete  life  cycle  of  the  Black  Rust 
and  a  fifth  kind  occurs.  The  uredospores 
and  teleutospores  occur  on  the  grains  or 
other  Grasses.  The  basidiospores  are  pro- 
duced by  the  teleutospores  and  no  host  is  required,  while  aecidio- 
spores occur  on  the  Barberry  bush. 

If  the  Black  Rust  must  have  all  of  the  stages  in  order  to 
propagate  from  year  to  year,  then  it  seems  that  there  should  be 
little  or  no  Black  Rust  in  regions  where  there  are  no  Barberry 
bushes,  but  such  is  not  the  case,  for  the  Black  Rust  occurs 
abundantly  in  fields  many  miles  away  from  Barberry  bushes. 
Just  how  it  gets  started  on  the  grains  in  localities  where  there 
are  no  Barberry  bushes  is  not  definitely  known.     It  was  once 


Fig.  355. —  Tc- 
leutospore having 
developed  the  pro- 
mycelia  bearing 
basidiospores  (s). 


BLACK    lU'ST   OK   ({RAIX    (PUCCINIA   (iUAMIMS) 


399 


Fig.  356.  —  Stage  of  the  Wheat  Rust  on  the  Barberry  bush,  Bcrbcris 
vulgaris.  Left,  leaf  of  Barberry,  showing  the  affected  areas  which  are  red- 
dish, much  thickened,  and  contain  many  cup-Uke  depressions;  right,  a  very 
much  enlarged  section  through  the  affected  area  of  the  leaf,  showing  one  of 
the  cups  (c)  with  chains  of  aecidiospores  ( X  200) .  The  very  small  spores  at 
(p)  are  the  spermatia  or  pycniospores. 


Fig.  357. — ^Diagram  showing  the  life  cycle  of  the  Wheat  Rust.  A, 
wheat  plants;  B,  barberry  bush;  u,  uredosporc;  t,  teleutospore;  s,  basidio- 
spores;  a,  aecidiospore. 


400  THALLOPIIYTES 

thought  that  the  basidiospcres  started  the  disease  directly  on  the 
Grass  host,  but  experiments  have  shown  that  they  will  not  grow 
on  this  host.  Experiments  have  also  shown  that  uredosporcs 
are  ordinarily  killed  b}^  freezing  weather  and  therefore  are  rarely 
able  to  live  over  winter  where  the  temperature  goes  much  below 
freezing.  It  has  been  suggested  that  some  hyphae  may  enter  the 
kernels  of  the  diseased  plants  and  remain  dormant  until  the  seed 
is  planted  and  then  infect  the  seedling,  but  this  theory  is  not 
generally  accepted.  Another  suggestion  is  that  the  wind  carries 
the  uredospores  northward  from  the  Southern  states  where  they 


Fig.  358.  —  Apple  affected  with  Cedar  Rust.     From  Technical  Bulletin  9, 
Virginia  Agr.  Exp.  Sta. 

are  able  to  live  over  winter.  It  is  also  proljable  that  the  aecidio- 
spores  may  be  carried  a  considerable  distance  by  the  wind  and 
thus  reach  grain  fields  not  in  the  immediate  vicinity  of  Barberry 
bushes.  Then  there  is  the  probability  that  the  disease  may  start 
on  the  wild  Grasses  growing  near  the  Barberry  bushes,  and  be 
passed  along  l^y  the  uredospores  from  one  patch  of  Grass  to  an- 
other until  grain  fields  far  away  are  reached. 

No  satisfactory  preventative  for  the  Black  Rust  has  been  dis- 
covered. We  are  not  able  to  control  the  spores.  It  is  generally 
believed  that  the  eradication  of  all  of  the  Common  Barberry 
bushes  would  do  much  toward  eliminating  this  Rust.     The  most 


CEDAR  APPLES  AND  APPLE  RUST  (GYMXOSPORANGIUM)      401 


hope,  however,  seems  to  be  in  breeding  and  selecting  varieties  of 
grains  which  can  resist  the  attack  of  the  Rust,  and  some  progress 
has  ah-eady  been  made  in  this  direction. 

Cedar  Apples  and  Apple  Rust  (Gymnosporangium).i  — There 
are  several  Rusts  belonging  to  this  group,  but  the  one  producing 
Cedar  Apples  and  the  Rust 
on  Apple  trees  is  the  most 
common  and  the  most  im- 
portant of  the  group.  It  is 
common  in  nearly  every 
region  where  Red  Cedars 
grow,  but  does  most  damage 
to  fruit  trees  in  the  Eastern 
and  Southern  states.  It  lives 
a  part  of  its  life  cycle  on  the 
Cedar,  producing  gall-like  en- 
largements on  the  branches, 
and  a  part  of  its  life  cycle  on 
the  Apple  tree  where  it  at- 
tacks the  leaves  and  fruit, 
often  causing  much  damage 
to  the  fruit  {Fig.  358).  It 
is  the  gall-like  enlargements 
on  the  Cedar  tree  that  are 
called  Cedar  apples,  although 
they  are  not  apples  at  all. 
In  Figure  359  are  shown 
Cedar  apples  as  they  appear 
in  the  winter.  In  the  spring 
gummy  branches  containing 
many  teleutospores  develop  on  these  galls  which  then  look  like 
the  one  shown  in  Figure  360.  The  teleutospores  produce  basidio- 
spores  which  are  blown  to  the  Apple  tree  where  they  start  the 

1  The  Cedar-Apple  Fungi  and  Apple  Rust  in  Iowa.  Bulletin  S4,  Iowa 
Agr.  Exp.  Sta.,  1905. 

The  Life  History  of  the  Cedar  Rust  Fungus  Gymnosporangium  juniperi- 
virginianae.     Ayinual  Report  22,  pp.  105-113,  Nebraska  Agr.  Exp.  Sta.,  1909. 

Apple  Rust  and  its  Control  in  Wisconsin.  Bulletin  2.57,  Wisconsin  Agr. 
Exp.  Sta.,  1915. 

The  Cedar  Rust  Disease  of  Apples  caused  by  Gymnosporangium  junipcri- 
virginianae  Schw.     Technical  Bulletin  9,  Virginia  Agr.  Exp.  Sta.,  1915. 


Fig.  359.  —  Cedar  Apples  on  the 
Cedar.  This  is  the  way  the  galls  look 
in  winter.  From  Bulletin  257,  Wiscon- 
sin Agr.  Exp.  Sta. 


402 


TIIALLOPHYTES 


disease  on  the  leaves  and  fruit.  Upon  the  Apple  tree,  the  aecidia 
stage  is  produced,  and  the  aocidiospores  are  able  to  attack  the 
Cedar  and  form  new  galls,  thus  completing  the  life  cycle  as  shown 
in  Figure  361. 

Pine  Tree  Blister-rust  (Cronartium  ribicola).  —  As  its  name 
suggests  this  Rust  attacks  Pine  trees.     It  was  introduced  from 

Europe  about  ten  years  ago 
and  has  now  become  a  seri- 
ous disease  in  i^his  countr5^ 
It  has  its  aecidial  stage  on 
Pines  with  five  leaves  in  a 
fascicle,  such  as  the  White 
Pine  and  Sugar  Pine,  and 
has  species  of  Ribes  (Goose- 
berries and  Currants)  as  the 
other  host.  In  this  Rust  the 
aecidial  stage  is  the  most  de- 
structive. The  mycelium  of 
the  aecidial  stage  kills  the 
cambium  and  inner  bark  of 
Pines,  thus  causing  the 
death  of  branches  and  some- 
times of  the  entire  tree. 
Both  uredospores  and  teleu- 
tospores  are  produced  on 
the  infected  Currant  and 
Gooseberry  bushes,  which 
are  apparently  very  little 
injured  thereby.  Pines  are 
infected  through  the  basid- 
iospores.  The  chief  means 
of  checking  the  spread  of 
the  disease  is  through  the  destruction  of  the  wild  Currant  and 
Gooseberry  bushes. 

The  damage  done  to  Pine  trees  is  serious  and  since  our 
Pine  forests  are  valued  at  many  millions  of  dollars,  it  is 
not  surprising  that  our  government  has  put  restrictions 
upon  the  importation  of  Pines  from  Europe  and  has  appro- 
priated large  sums  of  money  to  be  expended  in  checking 
this  disease. 


Fig.  360.  —  A  Cedar  Apple  which  has 
developed  the  gelatinous  branches  con- 
taining numerous  teleutospores.  The 
teleutospores  produce  sporidia  or  basidio- 
spores  that  attack  the  Apple  tree.  These 
gelatinous  branches  develop  in  the  spring 
after  a  rain  and  while  the  leaves  and 
shoots  of  the  Apple  are  young  and  easily 
attacked.  After  Bulletin  257,  Wisconsin 
Agr.  Exp.  Sta. 


SUMMARY   OF   BASIDIOMYCU<7rES 


403 


Asparagus  Rust.  —  Asparagus  is  often  attacked  by  a  Rust 
(Puccinia  Asparagi)  which  is  a  type  of  those  luiving  l)ut  one 
host.  The  uredospores,  teleutospores,  and  aecidiospores  all  occur 
on  the  Asparagus. 

Some  other  forms  of  Rusts  of  some  importance  occur  on  Clover, 
Alfalfa,  Beans,  Peas,  Beets,  Timothy,  Corn,  Peach  trees,  etc. 

Summary  of  Basidiomycetes.  —  Like  the  Ascomycetes  the 
Basidiomycetes  are  parasites  or  saprophytes  on  land  plants  and 
have  no  motile  spores.     The  Basidiomycetes  are  supposed  to 


spondia 
teleufospore    H       ^ 


aecidiospores 


Fig.  361.  —  Diagram  .showing  life  history  of  the  Cedar  Rust  Fungus.  A, 
Cedar  tree;  B,  Apple  tree.  The  sporidia  from  the  teleutospores  infect  the 
Apple  tree  and  the  aecidiospores  produced  on  the  Apple  foliage  during  summer 
reinfect  the  cedars.     From  Technical  Bulletin  9,  Virginia  Agr.  Exp.  Sta. 

have  been  evolved  from  the  Ascomycetes,  and  hence  are  farthest 
removed  from  the  Algae,  which  they  resemble  very  little. 

In  such  Basidiomycetes  as  the  Toadstools  and  Puffballs,  the 
most  highly  developed  sporophores  occur,  while  in  the  parasitic 
Basidiomycetes,  as  the  Smuts  and  Rusts,  the  mycelium  is  scat- 
tered through  the  host  and  is  only  visible  through  the  production 
of  spore  masses. 

Such  forms  as  the  Toadstools,  Mushrooms,  and  Puffballs 
reproduce  entirely  by  basidiospores,  while  in  the  reproduction 
of  the  Smuts  brand  and  basidiospores  are  involved,  and  in  the 
reproduction  of  Rusts  there  are  four  kinds  of  functional  spores 
—  uredo-,  teleuto-,  basidio-,  and  aecidiospores,  —  and  the  non- 
functional spermatia. 


404 


THALLOPHYTES 


Fungi  Imperfect!  (Imperfect  Fungi) 

All  Fungi  in  which  the  features  characteristic  of  the  Phycomy- 
cetes,  uVscomycetes,  or  Basidiomycetes  have  not  been  discovered 
in  then-  life  histories  are  classed  as  imperfect  Fungi.  It  is  a 
heterogenous  group,  containing  numerous  Fungi  varying  widely 
in  characteristics.  Investigators  think  that  most  of  them  are 
the  conidial  stages  of  Ascomycetes  in  which  the  Ascogenous 
.  stage  has  been  abandoned  or  has  not 

been  discovered.  Careful  investiga- 
tions have  already  discovered  that 
a  number  of  Fungi  which  have  been 
classed  as  imperfect  Fungi  have 
ascogenous  stages  and  are  therefore 
Ascomycetes.  As  investigations  go 
on  no  doubt  others  and  probably  all 
of  them  will  be  definitely  classified  in 
the  other  groups. 

The  spore  commonly  known  in  the 
group  is  the  conidiospore  and  the 
character  of  this  spore  and  the  way 
it  is  borne  are  the  chief  features  upon 
which  the  group  is  divided  into  nu- 
merous subdivisions. 
Among  them  are  many  disease-producing  forms,  a  large  number 
of  which  produce  serious  diseases  on  cultivated  plants.  The 
Early  Blight  of  the  Potato,  Leaf  Blight  of  Cotton,  Black  Rot  of 
the  Sweet  Potato,  Fruit  Spot  of  Apples,  one  of  the  Potato  Scabs, 
Apple  Blotch  shown  in  Figure  862,  and  numerous  other  diseases 
are  produced  by  these  Fungi. 
Some  special  books  on  Fungi : 

Harshberger,  John  W.     A  Text-Book  of  Mycology  and  Plant  Pathology. 
Stevens,  F.  L.     The  Fungi  which  Cause  Plant  Diseases. 
DuGGAR,  Benjamin  M.     Fungous  Diseases  of  Plants. 
Massee,  George.     Diseases  of  Cultivated  Plants. 
Massee,  George  and  Ivy.     Mildews,  Rusts,  and  Smuts. 


J 


Fig.  362. —Apple  affected 
with  Apple  Blotch  caused  by 
an  Imperfect  Fungus.  From 
BuUdin  lU,  Bureau  of  Plant 
Industry,  U.  S.  Dept.  of 
Agriculture 


CHAPTER  XVI 

BRYOPHYTES    (MOSS   PLANTS) 

Liverworts  and  Mosses 

General  Discussion.  —  In  the  study  of  the  Myxomycetes, 
Bacteria,  and  Fungi,  not  much  attention  was  given  to  evolution- 
ary tendencies,  for  these  groups  are  supposed  to  be  degenerate 
forms  and  have  contributed  nothing  of  importance  in  the  way  of 
evolution.  But  in  taking  up  the  study  of  the  Bryophytes,  we 
return  to  the  study  of  evolution  which  will  be  emphasized 
throughout  the  remaining  groups,  the  aim  being  to  see  how 
Flowering  Plants  could  have  originated. 

The  Bryophytes  include  two  large  groups  of  plants  —  Liver- 
worts ( Hepaticae)  and  Mosses  (Musci)  —  although  the  term 
refers  to  Mosses.  The  Mosses  are  more  conspicuous  and  more 
familiar  to  most  people  than  the  Liverworts,  but  they  are  no 
more  important  in  the  study  of  evolution. 

The  Bryophytes  are  of  practically  no  economic  importance. 
They  are  of  very  little  value  for  food  and  rarely  harm  other 
plants.  They  make  their  own  food  and  therefore  do  not  need 
to  prey  upon  other  plants.  The  only  reason  for  studying  them 
is  that  they  have  contributed  to  evolution,  and  a  knowledge  of 
them  is  necessary  for  an  understanding  of  the  higher  plants. 

The  Bryophytes  are  supposed  to  have  originated  from  the 
Algae,  and  the  advancements  made  by  the  Algae,  such  as  the 
establishment  of  multicellular  plant  bodies,  food-making  by 
photosynthesis,  development  of  gametes  and  sex  organs,  and  the 
differentiation  of  gametes  and  other  cells,  are  resumed  and  some 
of  them  carried  farther  by  the  Bryophytes. 

Most  Algae  live  in  the  water  while  the  Bryophytes  in  most 
part  live  on  the  land.  The  Bryophytes  are  considered  the  first 
and  most  primitive  land  plants.  The  Algae  are  exposed  to  water 
while  most  Bryophytes  are  exposed  to  the  drying  effects  of  the 
air.  Most  Algae  soon  die  when  removed  from  the  water  and 
exposed  to  the  air,  for  they  are  not  protected  against  loss  of  water 

405 


406  BRYOPHYTES   (MOSS   PLANTS) 

and  the  air  soon  diics  them  out.  To  live  on  land  a  plant  must  be 
protected  against  transpiration,  and  to  become  large  and  erect,  a 
plant  must  have  structures  for  connecting  it  to  the  ground  and 
a  stem  to  support  it  against  the  wind.  It  is  believed  that  the 
land  plants  came  from  Algae,  and  this  means  that  certain  Algae 
must  have  acquired  the  land  habit  and  in  so  doing  ceased  to  be 
Algae  and  became  Bryophytes.  One  can  imagine  that  this  trans- 
formation came  about  by  some  Algae  gradually  becoming  more 
and  more  adapted  to  living  on  the  shore,  where  they  were  often 
stranded,  until  finally  they  became  so  modified  as  to  be  fitted  to 
live  permanently  on  land. 

Liverworts 

The  Liverworts  are  thought  to  be  the  group  that  first  acquired 
the  land  habit,  for,  as  a  group,  they  are  less  complex  than  the 
Mosses  and  are  also  more  like  the  Algae  in  their  moisture  re- 
quirements. While  many  of  them  live  on  land,  there  are  some 
forms  which  still  live  in  water,  and  it  is,  therefore,  in  the  Liver- 
worts that  the  connection  between  water  forms  and  land  forms 
is  most  evident.  Even  most  of  those  Liverworts  that  live  on 
land  are  not  able  to  endure  dry  air  and  hot  sunshine,  for,  in  most 
part,  they  must  grow  in  places  that  are  moist  or  at  least  shaded. 
But  the  Liverworts  did  much  toward  establishing  the  land  habit, 
and  it  is  thought  that  our  strictly  land  plants  originated  from 
such  forms  as  the  Liverworts. 

The  plant  body  of  most  Liverworts  is  a  flat  body,  known  as 
a  ihallus,  but  in  some  forms  it  is  differentiated  into  stem-  and 
leaf-like  structures.  The  thallus  form  of  plant  body,  although 
varying  much  in  form  according  to  the  species,  is  usually  lobed 
and  often  branched.  Often  the  thalli  are  liver-shaped,  and 
their  shape  was  once  thought  to  signify  that  these  plants 
possess  special  virtues  in  the  cure  of  liver  diseases  —  whence 
the  name  Liverworts. 

The  thallus  forms  of  Liverworts  often  form  mat-like  coverings 
on  moist  soil  or  on  moist  rocks,  such  as  the  sides  of  a  cliff.  Those 
Liverworts  having  better  differentiated  plant  bodies  and  re- 
sembling Moss  commonly  grow  on  logs  and  tree  trunks  in  moist 
and  shady  woods.  There  are  about  4000  species  of  Liverworts 
and  they  vary  widely  in  complexity.     They  are  commonly  sub- 


THE  MARCHANTIAS 


407 


divided  into  three  orders  —  Marchantiales,  Jungermaniales,  and 
Anthocerotales. 

The  Marchantias.  —  The  Marchantiales  include  the  best 
known  Liverworts,  among  which  are  the  Marchantias,  the  most 
highly  specialized  Liverworts  of  this  order  and  the  family  after 
which  the  order  is  named.  The  Marchantia  common  in  the  north 
temperate  regions  is  Marchantia  polijmorpha.  It  grows  in  moist 
places,  often  occm-ring  abundantly  in  swampy  regions,  on  shaded 
river  banks,  and  on  protected  rocky  ledges.     It  often  gets  started 


Fig.  363.  —  A  female  and  a  male  plant  of  Marchantia  polymorphia,  show- 
ing the  external  features  of  the  plant  body  (about  natural  size).  The  two 
plants,  of  which  A  is  the  female  and  B  the  male  plant,  differ  most  noticeably 
in  the  character  of  the  gametophores  which  are  the  erect  stalks  with  expanded 
tops  (conceptacles)  on  which  the  sex  organs  occur,  r,  rhizoids;  c,  the  gemmae 
cups  which  are  concerned  with  vegetative  multiplication. 


in  greenhouses  where  it  develops  and  spreads  rapidly  on  moist 
soil  that  is  left  undisturbed.  Being  easily  obtained,  it  is  one  of 
the  Liverworts  most  commonl}'^  studied  in  botanical  laboratorieSo 

The  plant  body  is  shown  in  Figure  363.  The  flat,  lobed,  green 
plant  body  or  thallus  lies  prostrate  on  the  substratum.  Often 
the  plants  are  so  much  crowded  as  to  overlap,  and  form  aggrega- 
tions that  cover  the  substratum  like  a  carpet. 

Single  plants  are  often  several  inches  in  length  and  breadth, 
and  consist  of  a  number  of  layers  of  cells  in  thickness.     On  the 


408 


BRYOPHYTES    (MOSS   PLANTS) 


untler  surface,  cells  are  differentiated  into  thread-like  structures 
called  rhizoids,  which  attach  the  plant  to  the  sul)stratum.  In 
the  notches  about  the  margin  are  cells  which  function  like  the 
meristematic  cells  of  the  higher  plants,  and  thus  have  to  do  with 
the  addition  of  new  cells  wherel)y  the  growth  of  the  plant  is 
maintained.  The  cells  of  the  upper  region  of  the  thallus  are 
differentiated  into  an  epidermis,  which  affords  protection  against 


Fig.  .364.  —  Highly  magnified  crass  sections  of  a  thallus  of  jVIarchantia. 
A,  section  through  a  thick  portion  of  a  thallus,  showing  the  following  features: 
the  upper  epidermis  and  the  chlorenchyma  tissue  {chl)  just  beneath  divided 
into  chambers  by  partitions  (o) ;  the  layers  of  cells  (p)  between  the  chloren- 
chyma and  lower  epidermis,  giving  thickness  and  rigidity  to  the  thallus;  and 
the  lower  epidermis  with  rhizoids  (/()  and  scale-like  plates  of  cells  (6).  B,  sec- 
tion near  the  margin  of  the  thallus  and  more  highly  megnified,  showing  the 
following  features:  upper  epidermis  (o);  a  chamber  of  chlorenchyma  tissue 
(chl)  bounded  by  the  partitions  (s)  and  into  which  the  chimney-like  air  pore 
{sp)  opens;  the  lower  epidermis  (m);  and  two  layers  of  supporting  tissue  (p). 
From  J.  M.  Coulter,  originally  after  Goebel. 

evaporation,  and  into  tissues  which  utilize  the  air  and  sunlight 
in  manufacturing  carbohydrates.  Highly  magnified  .sections 
through  a  thallus  are  shown  in  Figure  364.  In  the  epidermis  are 
many  chimney-shaped  pores  which  permit  the  air  to  reach  the 
filaments  of  food-making  cells  in  the  chambers  beneath. 

On  the  thalli  shown  in  Figure  363  are  also  shown  some  small 
cups  and  some  erect  stalks  with  expanded  tops.     These  struc- 


THE   MARCHANTIAS  409 

tures  are  further  differentiations  of  tlie  plant  body,  that  are  not 
always  present,  occurring  only  during  periods  of  reproduction. 
The  erect  umbrella-shaped  structures,  which  are  upgrowths  of 
the  midrib  of  the  thallus,  bear  the  sex  organs  and  are  called 
gametophores,  while  the  cup-shaped  structures  have  to  do  with  a 
vegetative  method  of  reproduction  which  will  be  discussed  later. 

Since  the  plant  lives  spread  out  on  a  moist  substratum,  much 
of  the  plant  body  is  in  direct  contact  with  moisture  and  can 
absorb  water  and  minerals  directly.  The  rhizoids  are  not  roots 
and  are  of  very  little  service  in  supplying  water  and  mineral  salts. 
It  is  probable  that  they  do  nothing  more  than  hold  the  plant 
body  to  the  substratum.  The  filaments  of  cells  in  the  air  cham- 
bers on  the  upper  surface  are  well  provided  with  chloroplasts  and 
carry  on  active  photosynthesis  which  supplies  the  plant  with 
carbohydrates.  Many  of  the  other  cells  in  the  upper  region  of 
the  plant  body  have  some  chloroplasts  and  no  doubt  assist  some 
in  providing  food. 

In  addition  to  the  sexual  method  of  reproduction,  there  are 
two  ways  of  propagating  vegetatively  or  asexually.  As  the 
branches  of  the  thalli  develop  and  push  ahead,  the  older  regions 
die  away  and  soon  the  branches  become  isolated  and  form  sepa- 
rate plants.  This  is  known  as  a  vegetative  or  asexual  method  of 
reproduction  because  no  spores  or  sex  cells  are  involved.  An- 
other vegetative  method  occurs  in  connection  with  the  cups 
which  have  been  pointed  out  on  the  surface  of  the  thalH.  In 
these  cups  are  produced  small  plates  of  cells,  called  gemmae, 
which,  when  splashed  out  by  rain  and  suitably  located  on  a 
substratum,  grow  directly  into  new  plants.  These  vegetative 
methods  remind  one  of  the  propagation  of  Strawberries  by  run- 
ners, or  of  Geraniums  by  cuttings. 

The  sex  organs  are  produced  upon  the  umbrella-like  tops  or 
receptacles  of  the  gametophores.  On  the  under  surface  of  the 
much  lobed  receptacles  of  the  gametophores  of  the  female  plants 
(A,  Fig.  363)  occur  the  female  sex  organs  called  archegonin. 
When  a  thin  section  is  made  through  a  female  receptacle  and 
examined  under  the  microscope,  the  archegonia  are  seen  pro- 
jecting from  the  under  surface  as  shown  at  A  in  Figure  365. 
Each  archegonium  consists  of  many  cells  so  arranged  as  to  form 
a  long  hollow  neck  and  an  enlarged  hollow  base  called  venter,  in 
which  the  large  egg  is  located.     It  is  obvious  that  an  archegonium 


410  BRYOPHYTES   (MOSS  PLANTS) 

is  much  more  complex  than  the  oogonium  of  the  algae.  In  the 
less  lobed  receptacles  of  the  gametophores  of  the  male  plants  {B, 
Fig.  363)  occur  the  antheridia,  consisting  of  a  stalk  and  of  a  jacket 
of  cells  which  encloses  a  mass  of  sperms  as  shown  in  Figure  366. 


Fig.  365.  —  Highly  magnified  vertical  sections  through  the  expanded  tops 
or  receptacles  of  female  gametophores  of  Marchantia,  showing  the  sex  organs 
and  sporophytes.  A,  section  through  female  gametophore,  showing  the 
archegonia  (a),  each  of  which  consists  of  a  neck  and  an  expanded  base  called 
venter,  in  which  the  egg  (e)  is  located  B,  section  through  a  female  gameto- 
phore, showing  sporophytes  (s),  with  their  sporangia  {h),  stalks  (0,  foot  (/), 
and  also  showing  spores  {%)  escaping  from  the  sporangium  of  the  sporophyte 
at  the  left. 

Since  the  sperms  are  produced  on  one  plant  and  the  eggs  on  an- 
other, the  sperms  have  a  considerable  distance  to  be  carried  to 
the  eggs.  The  sperms  are  splashed  about  during  heavy  rains,  and, 
when  near  an  archegonium,  they  are  attracted  to  the  entrance  in 
the  neck  by  an  attractive  substance  which  diffuses  out  of  the 
archegonium.  The  sperms  swim  down  the  canal  in  the  neck  of 
the  archegonium  and  the  first  one  reaching  the  egg  fertilizes  it. 
The  fertilized  egg  or  oospore  remains  where  it  was  formed,  be- 
gins to  grow  and  divide  rapidly,  and  soon  produces  an  oblong, 
multicellular,  brownish  body  which  consists  of  a  stalk  that  is 
attached  to  the  receptacle  by  an  absorbing  organ  called  foot 
and  bears  at  the  other  end  a  sporangium  {B,  Fig.  365).     The 


THE  TWO   GENERATIONS 


411 


foot  extends  into  the  gametophore  and  absorbs  food  which  is 
supplied  to  the  elongating  stalk  and  developing  sporangium. 
In  the  sporangium  are  produced  numerous  spores  and  also 
elongated  twisted  cells  called  elaters,  which  assist  in  scattering 
the  spores.  When  the  spores  arc 
mature  the  sporangial  wall  opens 
and  the  spores  are  scattered.  When 
the  spores  fall  on  a  moist  substra- 
tum, they  germinate  and  produce 
new  thallus  plants  like  the  ones 
described. 

The  Two  Generations.  —  The  ob- 
long body  produced  by  the  fertilized 
egg,  and  consisting  of  foot,  stalk, 
and  sporangium,  is  regarded  as  a 
plant  within  itself.  When  fully 
mature  it  is  so  small  that  one  must 
look  closely  under  the  finger-like 
lobes  to  find  it.  It  doesn't  look 
much  like  a  plant,  since  it  is  so 
simple  and  depends  upon  the 
gametophore  for  food  and  water, 
but  it  is  this  plant  that  differenti- 
ates  and   becomes  the  conspicuous 

plant  body  of  the  higher  plants.  Since  it  produces  spores,  it 
is  called  a  spore  plant  or  sporophyte.  When  one  is  reminded 
that  a  Corn  plant  or  Apple  tree  is  all  sporophyte  excepting  some 
microscopical  structures  within  the  flowers,  then  the  significance 
of  this  small  sporophyte  of  the  Liverworts  in  relation  to  the 
origin  of  the  higher  plants  may  be  realized. 

It  is  obvious  that  if  this  little  sporophyte  is  regarded  as  a  plant, 
then  all  of  the  remainder  of  Marchantia  must  be  regarded  as 
another  plant.  This  other  plant  consists  of  all  that  has  been 
described  as  the  plant  body  of  Marchantia.  It  consists  of  the 
flat  prostrate  thallus  and  the  gametophores  with  the  sex  organs 
and  gametes.  Since  it  is  the  function  of  this  plant  to  bear 
gametes,  it  is  called  gametophyte. 

It  follows  then  that  the  complete  life  cycle  of  Marchantia  in- 
volves two  plants  or  generations  as  illustrated  in  Figure  867. 
The  gametophyte  generation  develops  from  a  spore  and  produces 


Fig.  366. —  Highly  magnified 
vertical  section  through  the 
expanded  top  or  conceptacle  of 
a  male  gametophore,  showing 
the  antheridia  (a)  imbedded  in 
the  gametophore  and  consist- 
ing of  a  short  stalk  and  of  a 
jacket  enclosing  numerous  cells 
which  form  sperms. 


412 


BRYOPHYTES    (MOSS   PLANTS) 


gametes,  while  the  sporophyte  generation  develops  from  the  fer- 
tilized egg  and  produces  the  spores.  It  is  obvious  that  this  is 
alternation  of  generations.  Some  of  the  higher  Red  Algae,  as 
illustrated  by  Polysiphonia,  have  an  alternation  of  generations 


Fig.  367.  —  A  diagram  showing  the  Hie  history  of  Marchantia.  Above 
the  line  (a)  is  the  gametophyte  generation  and  below  the  line  is  the  sporo- 
phyte generation,  p,  spore;  q,  spore  germinating  to  form  gametophyte; 
g,  mature  gametophytes;  o,  sex  organs;  t,  gametes;  /,  fertilized  egg  or  first 
cell  of  the  sporophyte;  m,  fertilized  egg  dividing;  «,  mature  sporophyte 
ready  to  shed  spores  which  are  the  first  cells  of  new  gametophytes. 


in  their  life  cycle,  but  in  Bryophytes  this  feature  is  so  well  estab- 
lished that  it  occurs  everywhere  in  the  group  and  is  so  evident 
that  it  was  in  the  Bryophytes  that  the  alternation  of  generations 
was  first  observed.  Alternation  of  generations  is  also  an  estab- 
lished feature  of  Pteridophytes  and  Spermatophytes  or  all  plants 
above  the  Bryophytes. 

One  of  the  interesting  features  in  connection  with  the  transition 
from  the  gametophyte  generation  to  the  sporophyte  generation 
is  a  peculiar  kind  of  cell  division  known  as  the  reduction  division. 
It  will  be  recalled  that  in  preparation  for  cell  division  the  chro- 
matin in  the  nuclei  of  cells  forms  into  a  definite  number  of  chro- 
mosomes, the  number  depending  upon  the  kind  of  plant.  Now 
reckoning  nuclear  content  in  terms  of  chromosomes,  it  is  obvious 
that  since  fertilization  is  a  fusion  of  the  nuclear  contents  of  a 
sperm  and  an  egg,  the  number  of  chromosomes  in  the  nucleus  of 


THE  TWO   GENERATIONS 


413 


the  fertilized  egg  is  double  that  of  the  sperm  or  egg.  It  follows 
that,  unless  the  number  of  chromosomes  is  reduced  somewhere 
in  the  life  cycle  of  the  plant,  each  generation  of  plants  would  have 
double  the  number  of  chromosomes  of  the  preceding  generation. 
This  doubling  of  the  chromosome  number  in  each  generation 


Fig.  368.  —  Diagrams  showing  the  difference  between  ordinary  cell  divi- 
sion and  the  reduction  division.  To  make  the  diagrams  easy  to  follow  only 
two  chromosomes  in  each  case  are  represented,  but  their  behavior  is  typical 
of  all  the  chromosomes  of  the  nucleus.  One  chromosome  has  been  blackened 
and  the  other  left  white  to  indicate  that  they  differ  in  that  one  consists  of 
chromatin  material  of  the  father  parent  and  the  other,  of  the  mother  parent 
of  the  individual  whose  cell  division  is  illustrated  by  the  diagrams.  The 
upper  diagram  illustrates  the  behavior  of  chromosomes  in  ordinary  cell  divi- 
sion, showing  the  chromosomes  at  a  soon  after  organization,  their  arrange- 
ment on  the  spindle  fibers  and  splitting  lengthwise  at  h,  the  separation  of  the 
longitudinal  halves  at  c,  and  the  formation  of  the  new  nuclei  at  d,  with  each 
new  nucleus  containing  a  longitudinal  half  of  each  of  the  original  chromo- 
somes. In  the  lower  diagram,  illustrating  the  behavior  of  chromosomes 
in  the  reduction  division,  the  chromosomes  are  paired  at  e,  arranged  on  the 
spindle  in  pairs  at  /,  separated  as  whole  chromosomes  at  g,  and  thus  each 
nucleus  at  h  receives  one  chromosome  of  the  pair  and  not  half  of  each  chro- 
mosome as  in  ordinary  cell  division. 


would  soon  result  in  a  disastrous  piling  up  of  chromosomes.  In- 
vestigations show  that  the  sporophyte  has  twice  the  number  of 
chromosomes  of  the  gametophyte,  but  that  the  spores  formed  by 
the  sporophyte  have  the  gametophytic  number.  The  transition 
is  made  in  the  mother  cells,  that  is,  in  the  cells  which  form  the 
spores,  and  by  these  cells  dividing  in  such  a  way  that  the  chrome- 


414  BRYOPHYTES   (MOSS   PLANTS) 

somes  are  so  distributed  that  each  daughter  cell  gets  only  half 
of  the  number  of  chromosomes  or  the  gametophytic  number. 
This  kind  of  cell  division  is  called  the  reduction  division  and 
simply  undoes  the  doubling  of  chromosomes  resulting  from  fer- 
tilization. The  diagrams  in  Figure  368  show  how  the  reduction 
division  differs  from  ordinary  cell  division.  Cytologically  the 
sporophyte  begins  with  the  fertilized  egg  and  ends  with  mother 
cells,  while  the  gametophyte  begins  with  the  spore  and  ends  with 
fertilization.  More  will  be  said  about  the  significance  of  the 
reduction  division  in  connection  with  heredity  where  it  has  an 
important  bearing. 

The  Riccias.  —  The  genus  Riccia,  which  is  often  regarded  as  a 
subdivision  of  the  Marchantiales,  includes  the  simplest  of  Liver- 
worts.    Some  of  them  are  almost  entirely  aquatic,  living  sub- 


Fia.  369.  —  One  of  the  Riccias,  the  simplest  of  Liverworts.     X  4. 

merged  or  floating  on  the  surface  of  the  water,  while  others  live 
spread  out  on  moist  soil.  The  plant  body  is  a  simple  thallus, 
smaller  and  not  so  well  differentiated  as  the  thallus  of  Marchantia. 
{Fig.  369.)  No  gametophores  are  developed  and  the  sex  organs, 
both  kinds  of  which  may  develop  on  the  same  plant,  occur  in 
grooves  along  the  ribs  of  the  thallus.  The  air  pores  are  not  well 
developed  and  sometimes  rhizoids  are  absent.  The  sporophyte, 
which  is  also  much  simpler  than  the  sporophyte  of  the  Mar- 
chantias,  lacks  a  foot  and  stalk,  and  thus  consists  of  only  a 
sporangium. 

When  the  sporophytes  of  the  Riccias  and  Marchantias  are 
compared,  it  is  obvious  that  much  more  of  the  fertilized  egg  has 
been  turned  into  spores  in  the  Riccias  than  in  the  Marchantias. 
In  the  Marchantias  much  of  the  cell  progeny  of  the  fertilized  egg, 
instead  of  forming  spores,  is  used  in  forming  a  foot,  stalk,  and 
elaters.     Such  a  diverting  of  the  cells  which  could  form  spores 


PORELLA 


415 


into  other  kinds  of  work  is  spoken  of  as  sterilization  of  sporog- 
enous  tissue.  One  can  now  see  that  a  sporophytc  could  become 
as  complex  as  a  Corn  plant  by  becoming  more  and  more  multi- 
cellular while  at  the  same  time  most  of  the  cells  were  used  in 
forming  structures,  such  as  roots,  stems,  and  leaves.  In  this 
way  sporophytes  became  more  and  more  com- 
plex until  the  highest  plant  forms  were  pro- 
duced. 

Porella.  —  This  Liverwort  belongs  to  the 
Jungermaniales,  which  order  contains  the 
largest  number  of  Liverworts. 

The  Jungermaniales  vary  widely  in  their 
moisture  requirements,  some  being  able  to 
live  in  dry  situations.  They  are  especially 
abundant  in  the  tropics  where  they  grow  on 
the  trunks  of  trees,  on  leaves  of  other  plants, 
and  on  the  ground.  Some  have  thallose  game- 
tophytes  like  the  Marchantiales,  while  others, 
known  as  foliose  forms,  have  gametophytes 
that  are  differentiated  into  leaf-  and  stem- 
Hke  structures  and  resemble  the  Mosses. 

Porella  is  one  of  the  foliose  forms  of  the 
Jungermaniales  and  is  common  on  the  trunks 
of  trees  and  fallen  logs  in  the  north  temperate 
regions.  The  character  of  the  gametophyte  is  shown  in  Figure  370. 
It  has  a  slender,  creeping,  branched,  stem-like  axis  bearing  two  hor- 
izontal rows  of  larger  leaves  on  the  dorsal  surface  and  one  horizontal 
row  of  smaller  leaves  on  the  ventral  surface.  Although  much  more 
differentiated  as  to  form,  the  gametophyte  of  Porella  is  much  less 
differentiated  as  to  tissues  than  the  gametophyte  of  theMarchantias. 

The  two  kinds  of  sex  organs  may  occur  on  the  same  plant  or  on 
different  plants.  The  Archegonia  occur  in  groups  on  the  ends 
of  short  lateral  branches.  The  antheridia  occur  in  the  axils  of  the 
leaves  of  certain  branches  which  can  be  identified  bj^  the  closely 
imbricated  leaves. 

The  sporophyte  has  a  long  stalk  and  the  sporangium  splits 
into  four  valves  which  spread  out  and  allow  the  spores  to  escape. 
There  is  more  sterilization  of  sporogenous  tissue  and  a  more 
definite  provision  for  the  shedding  of  spores  than  in  the  sporo- 
phyte of  the  Marchantias. 


Fig.  370.  —  A 
branch  of  Porella, 
a  foliose  Liverwort 
of  the  Jungermani- 
ales.    X  3. 


416 


BRYOPHYTES   (MOSS   PLANTS) 


Thus  as  compared  with  the  Marchantiales,  the  Jungermaniales 
have  gametophytes  more  differentiated  in  form  but  less  in  struc- 
ture, and  have  sporophytes  characterized  by  a  greater  sterilization 
of  sporogenous  tissue. 

Anthoceros.  —  Anthoceros  is  a  representative  of  the  Antho- 
cerotales  which  is  a  very  small  group  of  inconspicuous  Liverworts. 
Anthoceros  and  its  allied  forms  are  the  most  interesting  of  all 
Liverworts,  because  their  structure  suggests  the  steps  by  which 
Pteridophytes,  the  Fern  group,  could  have  originated  from  the 
Bryophytes.     Anthoceros  grows  spread   out  like  some   of  the 

Riccias  and  is  common  on  moist 
soil  in  north  temperate  regions 
{Fig.  371).  The  gametophyte  is 
a  simple  thallus,  much  simpler 
than  that  of  the  Marchantias. 
The  sex  organs  develop  in  sunken 
areas  on  the  top  surface  of  the 
thallus. 

The  remarkable  feature  is  the 
sporophyte,  which  differs  in  a 
number  of  ways  from  the  spo- 
rophytes of  other  Liverworts. 
In  the  first  place  the  sporo- 
phyte is  green,  which  means  that 
it  is  supplied  with  chloroplasts 
and  is  thereby  able  to  make 
food  for  itself,  although  it  has  to  depend  upon  the  gameto- 
phyte for  water  and  mineral  salts.  This  feature  suggests  the 
independent  sporophyte  of  the  Pteridophytes.  The  epidermis 
of  the  sporophyte  even  contains  stomata  for  allowing  the  air 
to  reach  the  green  tissues  beneath  as  in  the  leaves  of  higher 
plants.  Evidently,  if  this  sporophyte  had  roots,  it  could  live 
independently  of  the  gametophyte.  In  the  second  place  there 
is  a  core  or  central  axis  of  sterile  tissue  called  columella  extend- 
ing lengthwise  through  the  sporophyte,  and  bands  of  spore- 
forming  tissue  alternate  with  bands  of  sterile  tissue  around  this 
columella.  The  columella  is  a  characteristic  feature  of  Moss 
sporophytes,  and  in  this  way  the  Anthocerotales  relate  the 
Liverworts  to  Mosses.  If  one  imagines  the  bands  of  sterile 
tissue  which  alternate  with  the  bands  of  spore-forming  tissue 


Fig.  371.  —  Anthoceros,  showing 
a  gametophyte  {g)  bearing  sporo- 
phytes (s).     X  about  2. 


TRUE  MOSSES    (BRYALES)  417 

growing  out  so  as  to  form  leaves,  then  a  leafy  sporophyte  like 
those  of  Pteridophytes  would  be  fornied.  In  the  third  place 
there  is  a  meristematic  group  of  cells  at  the  base  of  the  sporophyte 
by  which  growth  and  production  of  spores  are  maintained  for  a 
period  of  time. 

Mosses 

General  Description.  —  In  general,  Mosses  do  not  need  so  much 
moisture  as  Liverworts  do,  and  are,  therefore,  more  generally 
distributed.  They  are  common  in  moist  places  and  some  inhabit 
bogs  and  streams,  but  Mosses  are  also  very  common  in  dry 
places.  They  live  on  tree  trunks,  logs,  stumps,  rocks,  soil,  and  in 
bogs  and  fresh  water.  In  fact  one  can  find  Mosses  nearly  every- 
where. They  often  mass  together  in  clumps  and  cushion-like 
masses  which  hold  water  much  like  a  sponge.  Many  Mosses, 
especially  those  growing  in  dry  places,  can  become  dried  out  and 
then  revive  when  they  become  moist  again.  The  Mosses  as  a 
group  have  better  differentiated  gametophytes  and  sporophytes 
than  the  Liverworts. 

The  Mosses  are  divided  into  three  groups,  Sphagnales,  Andrea- 
les,  and  Bryales.  The  Sphagnales  are  the  Sphagnums,  which  live 
in  bogs  where  the  accumulation  of  their  plant  bodies  forms  peat. 
The  Andreales  are  a  very  small  group  of  siliceous  rock  Mosses 
which  will  receive  no  further  discussion,  although  they  are  inter- 
esting because  they  present  a  combination  of  characters  which 
relate  them  to  the  Sphagnales,  Bryales,  and  also  to  Liverworts. 
The  group  containing  the  vast  assemblage  of  our  most  familiar 
Mosses  is  the  Bryales.  The  Bryales,  known  also  as  the  True 
Mosses,  are  the  most  highly  organized  of  the  Mosses. 

True  Mosses  (Bryales).  — The  most  conspicuous  part  of  the 
Moss  plant  is  the  ganictophyte,  which  looks  like  Figure  372.  It 
consists  of  a  leafy  stem  attached  to  the  substratum  by  rhizoids. 
In  some  Mosses  the  leafy  stem  is  prostrate,  but  in  many  it  grows 
erect.  The  leaves  of  the  Moss  plant,  like  the  leaves  of  the  foliose 
Liverworts,  are  quite  simple.  In  most  part  they  are  only  one 
cell  in  thickness.  They  have  no  stomata  and  no  palisade  or 
spongy  tissues.  Although  they  are  called  leaves,  it  is  obvious 
that  they  are  not  like  the  leaves  of  the  higher  plants.  But  their 
cells  contain  chloroplasts  and  they  make  carbohydrates  just  as 
the  leaves  of  the   higher   plants   do.     Stomata,    palisade,   and 


418 


BRYOPHYTES   (MOSS   PLANTS) 


spongy  tissues  cannot  occur  and  are  not  needed  until  leaves  be- 


come more  than  one  cell  in  thickness, 
simple  in  structure,  and  is  not  dif- 
ferentiated  into   the   tissues   which 
characterize    the    stems    of    higher 
plants. 

The  sporophytc  is  commonly  much 
larger  than  that  of  the  Liverworts 


The  stem  is  also  quite 


Fig.  372.  — Thegame- 
tophyte  of  a  Moss,  con- 
sisting of  stem-  (s)  and 
leaf -like  structures  {I), 
and  rhizoids  (r)  which 
attach  it  to  the  sub- 
stratum.    X  about  2. 


Fig.  373.  —  The  two 
generations  of  Moss,  g, 
gametophyte  genera- 
tion;  a,  sporophyte  gen- 
eration; s,  sporangium 
of  the  sporophyte. 


and  it  can  be  seen  usually  at  a  considerable  distance  projecting 
from  the  top  of  the  gametophyte.  A  plant  bearing  a  sporophyte 
looks  like  Figure  373. 


TRUE  MOSSES   (BRYALES) 


419 


Most  parts  of  the  Moss  absorb  water  and  salts  directly.  Even 
the  leaves  are  probably  able  to  absorb.  The  leaves  carry  on 
active  photosynthesis  and  supply  the  carbohydrates.  No  vascu- 
lar bundles  occur,  but  in  many  Mosses  there  are  strands  of  elon- 
gated cells  which  assist  in  conducting  and  distributing  the  foods. 
The  erect  habit  and  the  radiate  arrangement  of  the  leaves  on  the 
stem  enable  the  plant  to  make  the  best  use  of  light. 

Knowing  that  the  leafy  green  plant  is  the  gametophyte,  one 
knows  where  to  look  for  the  sex  organs.     They  are  produced  on 


Fig.  374.  —  The  sex  organ.s  of  Moss.  A,  highly  magnified  vertical  sec- 
tion through  the  apical  region  of  the  stem  of  a  gametophyte,  showing  arche- 
gonia  (a)  with  eggs  at  (c).  B,  a  similar  section  through  a  plant  bearing 
antheridia  (t).  Sperms  escaping  from  an  antheridium  and  one  sperm  much 
enlarged  are  shown  at  s. 


the  upper  end  of  the  stem  and  are  quite  well  surrounded  and  hid- 
den by  the  upper  leaves.  If  one  carefully  pulls  off  the  terminal 
leaves  from  plants  that  are  in  the  reproductive  condition,  the 
sex  organs  may  be  found.  They  stand  erect  on  the  stem  tip  and 
are  so  large  that  they  can  be  seen  with  a  magnifier  of  very  low 
power.  The  antheridia  can  sometimes  be  seen  without  any 
magnifier.  The  archegonia  are  flask-shaped  and  have  very  long 
necks,  while  the  antheridia  are  club-shaped  {Fig.  374)-  In  many 
Mosses  both  sex  organs  occur  on  the  same  plant,  but  in  the  one 
shown  in  the  Figure  they  occur  on  separate  plants.     The  male 


420  BRYOPHYTES   (MOSS  PLANTS) 

plants  of  some  Mosses  can  be  identified  by  a  small  terminal  cup 
in  which  the  antheridia  are  produced. 

The  antheridia  produce  numerous  swimming  sperms,  and,  when 
there  is  suitable  moisture,  the  sperms  reach  the  archegonia,  swim 
down  the  long  necks  into  the  venters,  and  fertilize  the  eggs. 
The  fertilized  egg  ])egins  to  grow  almost  immediately  after  fer- 
tilization, and  like  the  fertilized  egg  of  the  Liverworts,  it  develops 
in  the  place  in  which  it  was  formed.  By  rapid  growth  and  cell 
division,  it  soon  forms  a  spindle-shaped  body  with  one  end  called 
foot  pushing  into  the  stem  of  the  gametophyte  to  absorb  food, 


Fig.  375.  —  A   protonema   of   Moss  (X  50).     Buds   which   develop   leafy 
gametophores  are  shown  at  b. 

and  the  other  end  pushing  into  the  air,  forming  a  stalk  called  seta 
which  bears  a  sporangium  at  its  upper  end  in  which  the  spores 
are  produced.  As  the  sporophyte  develops,  the  venter  about  the 
young  sporophyte  and  also  the  neck  of  the  archegonium  enlarge. 
Finally  the  venter  is  ruptured  and  the  enlarged  archegonium  is 
carried  up  by  the  sporophyte,  forming  a  pointed  cap  on  the  top 
of  the  sporangium.  When  the  spores  are  shed  and  fall  on  a 
moist  soil,  they  produce  new  gametophytes.  However,  the 
spore  does  not  grow  a  leafy  plant  directly,  but  first  produces 
an  Alga-like  filament  which  branches  and  creeps  over  the 
substratum  {Fig.  375).  From  bud-like  structures  on  this  fila- 
ment, the   leafy  green   plants   grow,  thus   completing   the   life 


TRUE   MOSSES    (BR Y ALES) 


421 


cycle  as  shown  in  Figure  376.  The  Alga-Hke  filament  called 
protonema  is  comparable  to  the  thallus  of  the  Marchantias,  and 
the  leafy  plants  to  the  gametophores.  Although  the  leafy  plants 
or  gametophores  of  Moss  are  not  all  of  the  gametophyte,  they  are 
the  conspicuous  part  of  it,  the  protonemas  being  microscopic  in 
size.  One  protonema  may  produce  many  buds,  and,  therefore, 
many  gametophores. 

In  Moss  the  two  generations  are  more  noticeable  than  in  the 
Liverworts.  The  gametophytes  with  their  leafy  gametophores 
present  more  differentiation  than  is  the  rule  among  Liverworts. 


Fig.  376.  —  Diagram  of  the  life  cycle  of  Moss,  p,  protonemas  from 
which  the  gametophores  (g)  have  arisen;  a  and  b,  the  sex  organs  with  a 
sperm  shown  passmg  from  antheridium  to  archegonium;  c  sporophyte  which 
the  fertilized  egg  produces;  s,  spores  which  grow  new  protonemas  and  thus 
the  life  cycle  is  completed. 


The  sporophyte,  consisting  of  a  large  sporangium  supported  on  a 
long  stalk,  or  seta,  is  usually  quite  conspicuous.  It  is  more  multi- 
cellular and  has  carried  the  sterilization  of  sporogenous  tissue 
farther  than  the  sporophytes  of  most  Liverworts  have.  Not 
only  is  it  larger  and  more  multicellular,  but  it  also  shows  more 
differentiation  than  the  sporophytes  of  Liverworts.  The  seta 
is  so  differentiated  as  to  have  a  central  strand  of  elongated 
cells  for  conduction.  The  sporangium  of  the  Moss  sporophyte 
develops  at  its  top  a  special  lid-like  structure  (operculum)  for 
opening,  and  often  special  tooth-like  structures  (peristome)  are 
produced  just  under  the  lid  and  assist  in  scattering  the  spores. 


422  BRYOPHYTES   (MOSS  PLANTS) 

In  the  sporangium  there  is  a  columella  or  axis  of  sterile  tissue, 
and  in  the  sporangial  wall  air  spaces  and  filaments  of  green 
tissue  are  provided.  In  some  Mosses  the  base  of  the  capsule, 
called  apophysis,  is  devoted  to  food-making  rather  than  to 
the  formation  of  spores,  in  which  case  there  is  much  chlorophyll 
tissue  and  many  stomata  present.  This  feature  is  quite  impor- 
tant as  was  pointed  out  in  Anthoceros,  because  it  looks  forward 
to  the  independence  of  the  sporophyte;  for,  if  the  sporophyte 
can  make  carbohydrates  for  itself,  it  then  needs  only  roots  to 
absorb  water  and  mineral  salts,  in  order  to  live  independently  of 
the  gametophyte. 

The  gametophytes  of  the  Mosses  have  a  remarkable  power  of 
propagating  vegetatively.  Since  the  sperms  depend  upon  w^ater 
for  transportation  and  the  sex  organs  are  borne  above  the  moist 
substratum,  fertilization  rarely  occurs  in  some  Mosses,  which, 
therefore,  must  depend  largely  upon  vegetative  propagation. 
There  are  a  number  of  ways  by  Avhich  they  propagate  vegeta- 
tively. First,  by  the  isolation  of  branches  through  the  death  of 
the  older  axes;  second,  the  cells  of  the  protonema  sometimes 
separate,  become  restive,  and  later  from  each  resting  cell  a  new 
protonema  is  developed ;  third,  from  the  leaves  and  stems  of  the 
gametophore  new  protonemas  are  often  developed;  and  fourth, 
some  Mosses  develop  gemmae  which  are  commonly  borne  at  the 
summit  of  the  leafy  gametophore. 

The  Sphagnums  (Sphagnales).  —  The  genus  Sphagnum  in- 
cludes all  of  the  Mosses  of  this  order.  There  are  about  250 
species,  and  they  occur  mostly  in  temperate  and  arctic  regions. 
They  live  chiefly  in  bogs  and  are  commonly  called  Bog  or  Peat 
Mosses.  Their  slender,  branched,  leafy  gametophores  {Fig.  377) 
are  pale  in  color  due  to  the  fact  that  many  of  the  leaf  cells  as  well 
as  many  of  the  outer  cells  of  the  stem  are  empty  except  for  the 
water  and  air  Avhich  they  hold,  thus  containing  no  chloroplasts. 
It  is  due  to  the  ability  of  these  much  enlarged  empty  cells  to 
take  up  and  retain  water  by  capillarity  that  Sphagnum  retains 
moisture  so  well  when  used  in  germinating  boxes  or  for  moist 
packing  around  plants.  The  gametophores  are  commonly 
creeping,  turning  up  only  at  the  ends,  and  they  usually  form  close 
mats,  which  gradually  thicken  by  growth  above  and  eventually 
fill  up  bogs.  Due  to  the  indefinite  growth  at  the  tips,  gameto- 
phores may  attain  great  length  and  age.     In  bogs  where,  due  to 


THE   SPHAGNUMS    (HPilAGNALES) 


423 


the  lack  of  drainage,  organic  acids  accumulate  and  prevent  the 
action  of  Molds  and  Bacteria,  the  dead  remains  of  Sphagnum  and 
accompanying  plants  do  not  decay,  but  are  finally  transformed 
into  peat,  which  is  a  valuable 
fuel  in  some  countries,  espe- 
cially in  Ireland. 

Both  antheridia  and  arcli- 
egonia  are  stalked  and  are 
produced  on  branches.  The 
sex  organs  differ  from  those  of 
the  Bryales  in  their  develop- 
ment but  are  quite  similar  in 
appearance  when  mature. 

The  sporophyte  differs  from 
the  sporophyte  of  the  Bryales 
in  having  only  a  very  short 
seta,  which  is  only  a  neck  be- 
tween the  foot  and  the  capsule. 
In  connection  with  this  fea- 
ture there  occurs  another 
characteristic  feature  known 
as  the  pseudopodmm.  The 
pseudopodium,  which  replaces 
the  seta  in  function,  is  formed 
by  the  elongation  of  the  axis 
of  the  gametophore  just  be- 
neath the  sporophyte,  which 
is  thereby  carried  up  as  if  it 
were  on  an  elongating  seta. 
Another  peculiar  feature  of 
the    sporophyte   is    that    the 

columella  does  not  extend  entirely  to  the  top  of  the  spor- 
angium as  in  Bryales,  but  the  sporogenous  tissue  arches  over 
the  columella.  In  this  respect  the  sporophyte  is  like  that  of 
Anthoceros. 

When  the  spores  germinate,  instead  of  producing  a  filamentous 
protonema,  they  produce  a  flat  thallus  that  resembles  a  Liver- 
wort, and  from  buds  on  this  thallus  the  leafy  gametophores  arise. 
When  studied  in  detail  one  finds  that  Sphagnum  has  a  number 
of  features  characteristic  of  Liverworts  and  a  numl')er  that  are 


Fig.  377.  —  I'he  gametophyte  and 
sphorophyte  of  Sphagnium.  At  the 
left,  gametophyte  of  Sphagnium;  at 
the  right,  a  sporophyte  and  the  pseudo- 
podium; between,  a  vertical  section 
through  the  sporophyte,  showing  the 
short  rounded  foot,  the  short  neck-like 
seta,  and  the  globular  sporangium  in 
which  the  spores  are  borne  in  a  cavity 
forming  an  arch  over  the  columella. 


424  HKVOPIIYTKS    (MOSS   PLANTS) 

characteristic  of  the  Bivales,  while  it  has  some  that  belong  to 
neither.  It  is  often  cahed  a  synthetic  form,  for  it  combines  the 
characters  of  Liverworts  and  True  Mosses. 

Summary  of  Bryophytes.  —  Tlie  Bryophytes  show  progress 
over  the  Algae  in  a  numbei'  of  ways.  First,  the  Bryophytes 
established  the  land  habit,  which  meant  the  establishment  of  a 
plant  body  that  was  adapted  to  live  and  function  in  the  air  rather 
than  in  the  water.  In  establishing  the  land  habit  the  plant  body 
had  to  develop  tissues  to  protect  against  transpiration,  sex  cells 
had  to  be  jacketed,  and  sex  organs,  now  called  antheridia  and 
archegonia,  consequently  became  multicellular,  and  tissues  for 
utilizing  the  carbon  dioxide  of  the  air  and  sunlight  in  making  food 
had  to  be  provided.  Second,  although  alternation  of  generations 
is  quite  prominent  in  some  of  the  higher  Algae,  it  is  a  very  dis- 
tinct feature  throughout  the  Bryophytes.  Both  gametophyte 
and  sporophyte  generations  show  considerable  advancement 
from  the  simplest  Liverworts,  where  the  gametophyte  is  a  small 
flat  thallus  and  the  sporophyte  merely  a  sporangium,  to  the 
highest  of  the  Mosses,  where  there  is  a  leafy  gametophore  and  a 
sporophyte  with  a  Avell  developed  seta  and  a  sporangium  having 
an  operculum,  peristome,  columella,  aerenchyma,  and  food- 
making  tissues. 

It  should  be  noticed,  however,  that,  although  the  Bryophytes 
adopted  the  land  hal^it,  they  have  a  swimming  sperm  which  puts 
a  limit  on  the  size  of  gametophytes,  for  swimming  sperms  can 
travel  only  short  distances  and  only  when  water  is  present.  In 
Mosses  and  the  more  complex  Liverworts,  there  is  much  evidence 
that  a  large  percentage  of  the  sperms  are  not  able  to  reach  the 
archegonia.  But  the  spore,  since  it  is  protected  against  drying 
and  can,  therefore,  be  transported  by  the  wind,  puts  no  limit  on 
the  size  of  the  sporophyte.  This  means  that  the  higher  plants 
must  consist  chiefly  of  the  sporophytic  generation. 


CHAPTER  XVII 
PTERIDOPHYTES    (FERN   PLANTS) 

General  Discussion.  —  Ferns  are  much  larger  plants  than 
Bryophytes  and  consequently  are  much  better  known  by  the 
general  public.  In  the  woods  Ferns  are  common  and  often  they 
can  be  found  in  the  fields.  On  account  of  their  large,  attractive, 
feather-like  leaves,  they  are  common  house  plants  and  are  ex- 
tensively grown  in  greenhouses.  Most  Ferns  require  a  moist  or 
shady  region,  but  some  are  able  to  grow  in  dry  situations. 

In  studying  the  different  layers  of  rock  which  form  the  earth's 
crust,  many  Pteridophytes  are  found  preserved.  In  the  layer 
of  rock  from  which  coal  is  obtained,  Pteridophyte  fossils  are  very 
abundant.  These  fossils  show  that  Pteridophytes  were  at  one 
time  much  more  abundant  than  now.  Some  of  these  ancient 
forms  were  like  trees  in  size  and  resembled  Seed  Plants  more  than 
any  of  the  present  forms  do.  Although  the  forms  that  made 
most  advancement  toward  Seed  Plants  have  long  been  extinct, 
the  forms  which  now  exist  show  us  some  of  the  lines  along  which 
progress  was  made. 

In  beginning  the  study  of  Pteridophytes,  one  should  have  in 
mind  the  features  contributed  by  the  Bryophytes,  because  the 
Pteridophytes  are  supposed  to  have  come  from  forms  like  the 
Bryophytes,  although  we  are  not  able  to  connect  them  up  with 
any  of  the  existing  forms  of  Bryophytes.  From  forms  like  the 
Bryophytes,  the  Pteridophytes  inherited  the  land-habit.  Thej^ 
not  only  inherited  those  features  which  enable  plants  to  live, 
work,  and  reproduce  in  the  air,  but  they  have  improved  upon 
these  features,  so  that  in  general  they  are  better  fitted  to  li«ve  on 
land  than  most  of  the  Bryophytes.  They  have  the  alternation 
of  generation  which  the  Bryophytes  so  firmly  established  and 
have  carried  the  sterilization  of  sporogenous  tissue  so  far  that  the 
sporophyte  is  a  massive  and  well  differentiated  plant  body. 
Probably,  instead  of  speaking  of  it  as  sterilization  of  sporogenous 
tissue,  it  would  be  clearer  to  say  that  the  fertilized  egg  now  pro- 

425 


426  PTERIDOPHYTES   (FERN   PLANTS) 

duces  an  enormous  number  of  cells  which  go  to  form  vegetative 
tissues  of  various  kinds,  before  sporogenous  tissue  is  produced. 
Thus  by  delaying  the  formation  of  sporogenous  tissue,  the  sporo- 
phyte  of  Pteridophytes  has  become  more  and  more  massive  and 
at  the  same  time  with  its  larger  number  of  cells  has  formed  more 
kinds  of  tissues  than  occur  in  the  sporophytes  of  Br3^ophytes. 
It  is  the  sporophyte,  which  is  the  plant  that  we  call  the  Fern, 
that  is  the  conspicuous  generation  in  the  Pteridophytes.  The 
gametophytes  in  most  cases  are  quite  small  and  generally  simpler 
than  the  gametophytes  of  most  Liverworts,  In  passing  from  the 
Bryophytes,  where  the  sporophyte  is  small,  dependent,  and  rela- 
tively simple,  to  the  Pteridophytes,  where  the  sporophyte  is  so 
many  times  larger  and  differentiated  into  roots,  stems,  and  leaves 
so  that  it  lives  independently,  one  is  struck  with  the  big  jump 
between  the  two  groups.  In  the  absence  of  forms  to  bridge  over 
this  gap,  the  relation  between  the  Bryophytes  and  Pteridophytes 
is  obscure.  The  sporophyte  with  its  roots,  stem,'  and  leaves  is 
now  well  advanced  toward  Seed  Plants. 

Although  the  Pteridophytes  are  known  as  the  Fern  group, 
there  are  many  Pteridophytes,  of  which  Horsetails  and  Club 
Mosses  are  familiar  ones,  that  are  not  really  Ferns.  The  True 
Ferns  are  the  most  highly  specialized  and  much  the  largest  group 
of  the  Pteridophytes,  but  in  order  to  get  a  notion  of  the  most 
important  features  contributed  toward  Seed  Plants  by  Pterido- 
phytes, a  study  of  the  Ferns  should  be  followed  by  a  study  of 
some  other  groups  of  Pteridophytes. 

Filicales 

The  Filicales  are  composed  of  the  True  Ferns  and  the  Water 
Ferns.  The  latter  are  small  forms  living  in  the  water  or  mud  and 
are  supposed  to  be  an  aquatic  branch  of  the  True  Ferns.  Al- 
though the  Water  Ferns  present  some  features  of  interest  to 
special  morphologists,  they  will  receive  no  attention  in  this  brief 
discussion.  The  True  Ferns,  which  are  the  most  abundant  and 
familiar  of  all  Pteridophytes,  are  even  more  abundant  in  the 
tropics  than  in  the  temperate  regions.  In  the  tropics  the  sporo- 
phytes of  some  grow  so  large  as  to  be  called  Tree  Ferns. 

Sporophyte.  —  Since  the  gametophyte  is  very  inconspicuous, 
the  sporophyte,  or  the  plant  known  as  the  Fern,  is  the  only  genera- 


SPOROPHYTE 


427 


tion  of  the  Fern  which  people  in  general  know  (Fig.  378). 
There  is  much  range  in  size  of  Fern  sporophytes,  from  very  small 
plants  like  some  that  are  common  in  our  woods,  to  those  as  high 
as  a  man's  head,  and  to  the  Tree  Ferns  of  the  tropics  and  green- 
houses that  may  reach  a  height  of  forty  feet  or  more. 

The  stems  of  a  few  Ferns  are  erect  and  may  become  large  like 
the  trunk  of  a  tree,  as  the  Tree  Ferns  illustrate  {Fig.  379),  but  in 


Fig.  378.  —  A  fern  sporophyte.     r,  roots;    s,  stem;    a,  young  leaves  or 
fronds  iinfolding;  I,  mature  fronds.     After  Wossidlo. 


our  common  Ferns,  the  stems  remain  a  few  inches  under  the  sur- 
face of  the  ground  and,  as  they  elongate  and  push  horizontally 
through  the  soil,  leaves  are  produced  from  the  upper  and  roots 
from  the  lower  surface.  They  are  called  rootstocks  or  rhizomes, 
both  terms  referring  to  the  root-like  feature  of  growing  under 
the  ground. 

The  stems  of  Fern  sporophytes  are  wood}'  and  have  many  of 


428 


PTERIDOPHYTES    (FERN   PLANTS) 


the  structures  characteristic  of  the  stems  of  Seed  Phmts  and  are, 
therefore,  not  merely  stems  in  appearance  as  the  stem-Hke  struc- 
tures developed  by  the  gametophytes  of  Mosses  and  some  Liver- 
worts are.     It  remained  for  the  sporophyte  generation  to  develop 


Fig.  379.  —  A  Tree  Fen 


After  Bailey. 


a  real  stem.  At  the  tip  of  the  Fern  sporophyte  there  is  a  meriste- 
matic  region  which  by  the  rapid  growth  and  division  of  its  cells 
elongates  the  stem.  Just  behind  the  advancing  tip  new  roots 
and  leaves  are  developed  and  stem  tissues  are  formed.  A  cross 
section  of  a  stem,  as  shown  in  Figure  380,  shows  an  epidermis, 
cortex,  vascular  cylinder,  and  pith  —  tissues  characteristic  of 
the  stems  of  Seed  Plants. 

The  roots  too  are  true  roots  and  are  not  simple  structures  like 
the  rhizoids  of  gametophytes.  They  have  a  root  cap,  region  of 
growth  and  elongation,  epidermis,  root  hairs,  cortex,  and  vascu- 
lar cylinder,  thus  having  the  features  characteristic  of  the  roots 
of  Seed  Plants. 

The  leaves,  although  true  leaves,  are  generally  called  fronds,  a, 
term  formerly  applied  to  them  because  they  were  considered  a 
combination  of  leaf  and  stem.  Fern  leaves  are  usually  much 
branched  and  are  easily  identified  by  the  way  their  veins  branch 
and  by  the  way  they  develop  in  the  spring.  Their  veins  Ijranch 
by  forking;  that  is,  a  vein  divides  into  two  veins  of  equal  size 


SPOROPHYTE 


429 


{dichotomous  branching);  aiul  the  leaves  develop  in  the  spring 
by  unrolling  from  the  base,  much  like  unrolling  a  bolt  of  cloth, 
until  their  final  length  is  reached  {circinate  vernation) .  They  have 
epidermis,  stomata,  and  chlorenchyma  or  food-making  tissue, 
and  through  their  veins  run  well  developed  vascular  bundles. 


Fig.  380.  —  A  cross  section  of  a  Fern  stem,  showing  the  epidermis  (e),  the 
cortex  (c),  the  vascular  cyHnder  {v),  and  the  pith  (/j). 


The  s-porangia  occur  in  the  rusty  looking  spots,  called  sori 
(singular  sorus),  which  are  formed  at  certain  times  on  the  uncUn- 
surface  of  the  leaves  {B,  Fig.  381).  Each  sorus  has  a  membrane- 
like covering  called  indusium,  under  which  the  sporangia  are 
protected.  By  making  a  thin  cross  section  of  a  leaf,  so  that  the 
section  passes  through  a  sorus,  the  sporangia  then  appear  under 
the  low  power  of  the  microscope  as  shown  at  C  in  Figure  881 .  A 
number  of  sporangia  occur  in  a  sorus,  but  the  number  varies  in 
different  Ferns.  The  sporangia  are  usually  stalked  and  flattened, 
and  around  the  margin  there  is  a  row  of  heavy  walled  cells  form- 
ing the  annulus,  which  assists  in  oj^ening  the  sporangia  and 
scattering  the  spores  (D,  Fig.  381). 


430 


PTERIDOPHYTES    (FERN   PLANTS) 


The  character  of  the  sporangia  and  the  way  they  are  borne  vary 
much  in  different  Ferns  and  are  much  used  in  the  classification  of 

Ferns.  In  the  lowest  group 
of  the  True  Ferns  the  spor- 
angia are  borne  in  syn- 
angia,  which  are  apparently 
composed  of  united  spor- 
angia. In  some  Ferns  the 
sporangia  are  borne  singly. 
In  some  the  sori  have  no 
true  indusium,  but  the  edge 
of  the  leaf  folds  over  and 
protects  the  sporangia. 
Then  in  the  shape  of  the 
sporangia,  presence  or  ab- 
sence of  an  annulus,  the 
location  of  the  annulus, 
and  in  the  numlier  of 
spores  borne  in  a  sporan- 
gium, there  are  important 
differences  among  Ferns. 
Again  there  are  two  ways 
in  which  sporangia  begin 
their  development.  In 
some  Ferns,  known  as 
eusporangiates,  both  epi- 
dermal and  sub-epidermal 
cells  of  the  leaf  are  involved 
in  forming  the  sporangia, 
while  in  Ferns,  known  as 
leptosporangiates,  the  spor- 
angia are  formed  entirely 
from  the  epidermal  cells  of 
the  leaf. 

In    some    of    the    True 

Ferns  the  sporangia  are  not 

borne  on  ordinary  leaves, 

in   which   case    the   sporo- 

phyte  is  differentiated  into  vegetative  and  spore-bearing  regions. 

Sometimes  some  of  the  leaflets  are  devoted  entirely  to  l^earing 


Fig.  381.  —  A  sporophyte  and  spore- 
producing  structures  of  a  True  Fern.  A, 
a  Fern  sporophyte,  showing  roots  (r), 
stem  (sO,  and  a  leaf  (0  (X  about  ^).  B, 
an  enlarged  view  of  the  under  surface  of 
a  Fern  leaf,  bearing  sori  (so).  C,  highly 
magnified  section  through  a  Fern  leaf 
and  sorus,  with  section  of  leaf  shown  at 
I,  sporangia  at  sp,  and  indusium  at  i.  D, 
a  much  enlarged  view  of  a  sporangium, 
showing  annulus  a  and  method  of  opening 
to  allow  the  spores  (s)  to  escape. 


GAMETOPHYTE 


431 


spores  as  in  the  Interrupted  Fern  (Osmunda  Claytonia)  {Fig.  382). 
In  some  like  the  Sensitive  Fern  {Onoclea  sensibiUs),  common  along 
roadsides  and  in  wet  meadows,  there  are  two  distinctly  different 
kinds  of  fronds,  one  of  which  is  entirely  devoted  to  bearing  spores 
and  the  other  entirely  to  vegetative 
work  (Fig.  383).  This  separation  of 
spore-bearing  and  vegetative  tissues 
is  adhered  to  more  closely  in  some 
other  Pteridophytes  than  in  the 
True    Ferns,   and   it    is    a    feature 


Fig.  382.  —  a  portion  of  a  leaf  of  the 
Interrupted  Fern  {Osmunda  Claytonia), 
showing  a  pair  of  vegetative  leaflets  above 
and  below  and  between  them  two  pairs  of 
spore-bearing  leaflets. 


Fig.  383.  — The  Sensitive 
Fern  {Onoclea  sensibiUs), 
showing  a  vegetative  frond 
at  the  left  and  a  spore-bear- 
ing frond  at  the  right. 


of  considerable  significance  because  it  is  characteristic  of  Seed 
Plants. 

Gametophyte.  —  When  the  spores  are  shed  and  fall  in  moist 
places,  the  protoplasm  breaks  the  spore  wall  and  begins  the  de- 
velopment which  results  in  the  production  of  a  gametoph>i;e. 
In  True  Ferns  a  short  tube  with  one  or  more  rhizoids  at  the  spore 


432 


PTERIDOPHYTES    (FERN   PLANTS) 


end  is  first  produced.     The  development  of  this  tube,  called  germ 
tube,  is  germination.     The  germ  tube  soon  reaches  its  full  length, 
and  then  it  begins  to  broaden  at  the  outer  end  and  a  tiny,  green, 
heart-shaped  gametophyte  is  produced  (Fig.  384). 
y>fvn-v-.  The  gametophyte  resembles  the  thallus  of  the 

-^"^"^^^^/K^frr^  simplest  Liverworts.  When  mature  it  has  a 
cushion-like  central  axis  where  the  rhizoids  and 
sex  organs  are  developed,  and  wing-like  margins 
consisting  of  a  single  layer  of  cells.  The  game- 
tophyte is  called  a  prothullus,  the  term  referring 
to  the  fact  that  it  is  thallus-like  in  form  and 
precedes  the  sporophyte  in  reproduction.  In  and  around  Fern 
beds  in  greenhouses  Fern  gametophytes  are  quite  common  on  the 


Fig.  384.— 
Three  Fern 
gametophytes 
shown  about 
natural  size. 


Fig.  385.  —  An  enlarged  view  of  the 
under  surface  of  a  Fern  gametophyte, 
showing  the  archegonia  («),  the  antheridia 
(b),  and  the  rhizoids  (r). 


Fk;.  386.  —  A  Fern 
gametophyte  {g)  bearing 
a  young  sporophyte  [s) 
with  leaf  at  I  and  root 
at  V. 


damp  walls,  damp  soil,  and  on  the  sides  of  flower  pots.  Oc- 
casionally they  can  be  found  out  of  doors  about  Ferns  growing 
in  moist  shady  places.  They  lie  flat  on  the  substratum,  and  the 
sex  organs  are  borne  underneath  where  there  is  moisture  for  the 


GAMETOPHYTE 


433 


swimming  sperms  {Fig.  385).  The  chimnoy-shaped  archegonia 
are  near  the  notch  of  the  prothallus,  and  the  globular  anther- 
idia  are  in  the  region  of  the  rhizoids.  In  some  Ferns  the  male 
and  female  sex  organs  are  on  different  gametophytes. 

The  sperms  are  active  swimmers  and  reach  the  egg  by  swim- 
ming down  the  neck  of  the  archegonium  which,  like  the  arche- 
gonia of  Bryophytes,  opens  at  the  top  when  the  egg  is  ready  for 
fertilization.  From  the  neck  of  the  archegonium,  a  substance  is 
also  discharged,  which  chemically  attracts  the  sperms. 


Fig.  387.  —  A  diagram  of  the  life  cycle  of  a  Fern.  A,  sporophyte  bearing 
sori  in  which  the  sporangia  occur.  B,  a  gametophyte,  a  product  of  a  spore 
and  the  generation  bearing  the  gametes,  the  sperms  of  which  are  shown 
passing  from  the  antheridia  to  the  archegonia.  C,  gametophyte  bearing  a 
sporophyte,  which  soon  becomes  independent  andHike  the  one  at  A . 


The  fertiUzed  egg  immediately  grows  into  a  sporophyte,  which 
lives  on  the  gametophyte  only  until  it  has  roots  and  leaves 
sufficiently  developed  to  support  itself  {Fig.  386).  After  the 
sporophyte  reaches  maturity,  sori  are  developed  and  the  life  cycle 
is  completed  {Fig.  387) .  Among  a  group  of  gametophytes  one 
usually  finds  sporophytes  in  various  stages  of  development  and 
greenhouse  attendants  sometimes  collect  and  pot  the  j^oung 
sporophytes  growing  in  unfavorable  places,  so  that  they  mature 
and  thereby  increase  their  stock  of  Ferns.     Usually,  however, 


434 


PTERIDOPHYTES   (FERN   PLANTS) 


Ferns  are  propagated  vegetativc^y  in  greenhouses,  and  out  of  doors, 
where  conditions  are  usually  unfavorable  for  the  development  of 
their  delicate  gametophytes,  many  Ferns  propagate  almost  en- 


FiG.  388. 


A  Moonwort  {Bolrychium  Virginianum).     X  about  k. 


tirely  vegetatively.  Some  propagate  by  runners,  many  by  the 
branching  and  segmenting  of  the  rhizome,  some  by  buds  which 
fall  from  the  leaves  to  the  ground  where  they  develop  new  plants, 
and  some  by  the  leaves  bending  over  and  taking  root  at  their  tips. 


EQUISETALES   (HORSETAILS)  435 

Some  Plants  Resembling  True  Ferns. — Some  plants  which 
resemble  the  True  Ferns,  although  they  belong  to  another  group, 
are  the  Botrychiums  or  Moonworts  that  are  common  in  the  woods 
{Fig.  388).  They  have  an  underground  stem  which  sends  up 
leaves  that  have  a  finely  divided  vegetative  portion  and  a  spore- 
bearing  portion  that  much  resembles  clusters  of  small  grapes. 


Fig.  389.  —  A  section  through  the  tuber-like  gametophyte  of  Botrychium, 
showing  one  archegonium  and  a  number  of  antheridia  in  the  upper  surface. 
X  about  10. 

It  is,  however,  in  their  gametophyte  generation  that  they  differ 
most  from  True  Ferns.  Their  gametophytes  are  tuberous  sub- 
terranean structures  bearing  the  sex  organs  on  the  upper  surface, 
and  associated  with  the  gametophytes  there  is  always  an 
endophytic  Fungus   {Fig.   389). 

Equisetales  (Horsetails) 

In  ancient  times,  as  shown  by  their  fossils  in  coal  and  other 
kinds  of  rock,  the  Equisetales  were  very  abundant,  but  the  only 
surviving  group  is  the  Horsetails.  Their  slender  stems,  often 
called  Joint  Grass,  are  common  in  meadows,  in  moist  places  in 
the  woods  and  along  roadsides.  There  are  about  25  species  of 
Equisetum.  There  is  Equisetum  palustre  common  in  swamps, 
Equisetum  pratense  and  Equisetum  arvense  common  in  meadows 
and  fields,  and  so  on.  Those  growing  in  meadows  and  fields 
are  often  troublesome  weeds.  They  are  widely  distributed  over 
North  America  and  also  occur  on  other  continents.  They  range 
in  height  from  a  few  inches  to  several  feet.  It  is  reported  that 
one  form  in  the  West  Indies  and  Chili  sometimes  reaches  a  height 
of  40  feet,  but  in  our  region  3  or  4  feet  is  a  good  height.  The 
Equisetums  are  also  called  Scouring  Rushes  because  their  stems 
contain  silica  which  is  used  in  making  scouring  powders. 


436 


PTERIDOPHYTES    (FERN   PLANTS) 


Sporophyte.  —  The  sporophyte  consists  of  a  horizontal,  much 
branched,  underground  stem  from  which  two  kinds  of  aerial 
branches  or  shoots  arise  {Fig.  390).  One  kind  of  shoot  bears 
spores  and  is  called  a  fertile  shoot,  while  the  other  kind  does  only 
vegetative  work  and  is  called  a  sterile  shoot.     Both  kinds  of 


Fig.  390.  —  Equisetum  arvense.  A,  a  portion  of  the  underground  stem 
with  two  fertile  or  spore-bearing  shoots,  each  of  which  bears  a  strobilus  (d) 
(X  h)-  B,  a  portion  of  a  sterile  or  vegetative  shoot  (X  j).  C,  asporophore, 
showing  the  stalk  and  umbrella-like  top  on  the  under  surface  of  which  are  the 
sporangia  (e)  (X  6).  Below,  at  the  right,  are  shown  spores,  one  with  elaters 
coiled  about  the  spore  and  the  other  with  elaters  uncoiled  (X  about  15). 


shoots  are  formed  under  the  ground  in  the  fall  in  most  Equise- 
tums  and  are  thus  ready  to  elongate  and  appear  above  ground 
early  in  the  spring.  On  both  kinds  of  shoots  the  leaves  are  mere 
scales,  which  are  so  joined  as  to  form  a  sheath  at  each  node.  The 
sterile  shoots  produce  whorls  of  slender  branches  at  the  nodes  and 
are  so  finely  branched  as  to  resemble  a  horse's  tail  —  whence  the 
name  Horsetails.     The  food  is  made  by  the  green  cortex  of  the 


GAMETOPHYTES  437 

aerial  shoots  in  the  epidermis  of  which  are  stomata  through 
which  carbon  dioxide  and  oxygen  reach  the  cortex. 

The  fertile  branch  commonly  appears  first  in  the  spring,  and 
in  some  common  forms  of  Equisetum  bears  no  side  branches, 
thus  having  only  whorls  of  scale-like  leaves  at  the  nodes.  At  the 
apex  of  the  fertile  branch  is  borne  the  strobilus  (plural  strobili) 
which  is  so  named  because  of  its  resemblance  to  a  cone  such  as 
occurs  in  Pines  (A,  Fig.  390).  The  strobilus  consists  of  a  central 
axis  (the  prolongation  of  the  axis  of  the  branch)  to  which  are 
attached  the  stalked  shield-shaped  structures  or  sporangiophores, 
so  named  because  they  bear  sporangia  (C,  Fig.  390).  Some  re- 
gard the  sporangiophores  as  modified  leaves  and,  therefore,  call 
them  sporophylls,  which  means  spore-bearing  leaves,  but  until 
their  relation  to  leaves  is  definitely  determined,  sporangiophore 
is  the  safer  term.  Under  the  shield-shaped  top  of  the  sporan- 
giophores are  borne  the  sporangia,  ranging  from  five  to  ten  in 
number  on  each  sporangiophore.  The  spores  are  provided  with 
ribbon-like  appendages,  called  elaters,  which  become  entangled 
and  thus  cause  the  spores  to  fall  in  clumps.  The  spores,  although 
alike  in  size,  are  physiologically  different,  for  some  of  them  pro- 
duce only  male  while  others  produce  only  female  gametophytes. 
In  some  species  of  Equisetum  the  fertile  branch  dies  after  the 
spores  are  shed,  but  in  others  the  strobilus  falls  off  and  the  branch 
continues  to  elongate,  becomes  green,  and  makes  food  during  the 
remainder  of  the  growing  season. 

There  are  two  notable  features  presented  by  the  sporophytes 
of  the  Equisetums.  One  is  the  differentiation  of  the  aerial  por- 
tion of  the  stem  into  sterile  and  fertile  shoots.  The  second  is  the 
aggregation  of  sporogenous  tissue  into  a  strobilus.  The  sterile 
branch  is  a  means  by  which  sporangia  can  be  elevated,  so  that  the 
spores  are  in  a  good  position  to  be  scattered.  The  strobilus  is 
supposed  to  be  the  forerunner  of  the  flower,  which  likewise  is  a 
structure  consisting  essentially  of  aggregates  of  sporogenous  tis- 
sue, for  the  pollen  grains  are  spores,  and  also  in  the  ovules  there 
are  spores  developed. 

Gametophytes.  —  In  the  Equisetums  the  gametophytes  are 
much  more  reduced  than  in  the  True  Ferns  {Fig.  391).  They  are 
so  small  that  one  needs  a  lens  to  identify  them.  Unless  conditions 
are  very  favorable,  they  are  not  able  to  survive  out  of  doors,  and 
consequently  the  Equisetums  are  propagated  principally  vegeta- 


438  PTERIDOPHYTES   (FERN   PLANTS) 

tively.  The  gametophytes  are  small,  green,  ribbon-like  bodies  and 
lie  flat  on  the  surface  of  the  substratum.  The  male  gametophyte 
is  the  smaller  and  is  one  cell  in  thickness.  It  bears  the  antheridia 
on  the  tips  of  the  lobes  or  on  the  margin.     The  female  gametophyte 


Fig.  391.  —  The  gametophytes  of  Equisetum  arvense.  A,  female  gameto- 
phyte, showing  one  archegonium  {ar)  (X  about  20).  B,  male  gametophyte 
with  four  antheridia  showTi  {$)  {X  about  40). 

forms  a  cushion,  a  number  of  cells  in  thickness,  on  the  upper  sur- 
face of  which  the  archegonia  are  borne. 

The  multiciliate  sperms,  after  being  set  free  from  neighboring 
male  gametophytes,  swim  to  the  archegonia  and  down  their  necks 
to  the  eggs.  The  fertihzed  egg  begins  to  develop  immediately 
and  continues  until  a  new  sporophyte  is  formed,  and  the  hfe  cycle 
is  thus  completed. 

Lycopodiales  (Club  Mosses) 

About  one-eighth  of  the  hving  Pteridophytes  are  Club  Mosses. 
They  are  commonly  divided  into  four  groups  —  Lycopodium 
Phylloglossum,  Selaginella,  and  Isoetes  —  but  a  study  of  the 
Lycopodiums  and  the  Selaginellas  will  serve  to  give  a  general 
notion  of  the  Club  Mosses. 

The  Club  Mosses,  although  not  IMosses  at  all,  get  their  name 
from  their  Moss-like  stem  and  their  club-shaped  appearance  due 
to  the  large  terminal  strobili  which  some  have. 

Lycopodium.  —  There  are  several  hundred  species  of  Lycopo- 
diums, and  they  are  widely  distributed,  occurring  in  both  hemi- 
spheres and  from  the  torrid  to  the  frigid  zones.  They  prefer 
shady  places  and  some  are  aquatic. 


SPOROPHYTE 


439 


Sporophyte.  —  The  sporophytes  vary  considerably  in  the  dif- 
ferent species,  but  consist  of  a  stem  simple  or  branched,  bearing 
numerous  small  leaves  (Fig.  392).  In  numerous  species  com- 
mon in  temperate  America  the  stems  trail  over  the  ground. 
These  species  are  often  used  for  decorations  at  Christmas  time 
and  are  called  Ground  Pines,  probably  from  the  appearance  of 
their  foliage,  although  they  are  not  Pines  at  all. 

One  of  the  notable  features  of  the  sporophyte  has  to  do  with  a 
suggestion  as  to  the  origin  of 
the  strobilus.  In  the  simplest 
forms  all  leaves  are  alike  and 
sporangia  occur  in  the  axils  of 
the  leaves  on  most  any  part 
of  the  stem.  These  leaves  do 
the  vegetative  work  and  in 
addition  are  sporophylls  in  so 
far  as  they  bear  sporangia. 
In  the  more  advanced  sporo- 
phytes of  Lycopodium  only 
certain  leaves  bear  sporangia, 
and  these  leaves  differ  consid- 
erably in  form  as  well  as  in 
function  from  the  other  leaves. 
They  are  located  at  the  top  of 
the  stem,  forming  the  close 
aggregation  or  strobilus.  In 
such  forms  it  is  obvious  that 
there  are  two  distinct  kinds  of 
leaves — sporophylls  and  vege- 
tative leaves.  In  intermedi- 
ate forms  one  can  find  sporo- 
phytes in  which  the  leaves  are 

all  alike  but  some  bear  sporangia  while  some  do  not,  and  often 
leaves  bearing  rudimentary  sporangia  can  be  found.  These 
facts  have  suggested  that  all  leaves  were  at  first  spore-bearing  and 
that  foliage  leaves  are  sterilized  sporophylls.  According  to  this 
theory,  the  simplest  condition  is  one  in  which  all  leaves  bear 
sporangia,  and  the  differentiation  of  foliage  leaves  and  sporo- 
phylls came  about  by  sterilizing  the  leaves  from  below  until  the 
spore-bearing  leaves  were  finally  limited  to  the  top  of  the  stem. 


Fig.  392.  —  Lycopodium  complana- 
tum,  showing  vegetative  branches  and 
clusters  of  terminal  strobili  ( X  1)  •  At 
the  left  of  the  strobiU  is  an  enlarged 
view  of  a  sporophyll  showing  the  spor- 
angium. Below  the  sporophyll  are 
shown  some  spores  highly  magnified. 
Redrawn  from  Britton  &  Brown. 


440 


PTKHIDOPHYTES    (FERN    PLANTS) 


The  strobilus,  therefore,  arose  as  a  result  of  (liffercntiating  the 
leaves  in  function  and  aggregating  the  sporophylls.  Differing  in 
function,  sporophjdls  and  vegetative  leaves  would  come  to  differ 
in  form.  One  can  see  considerable  advantage  in  this  to  the  plant. 
It  permits  a  large  amount  of  leaf  tissue  to  be  devoted  entirely  to 
the  manufacture  of  food,  while  the  sporophylls,  since  they  are 
not  depended  upon  for  food,  can  be  nmch  crowded,  and  as  a  result 
many  spores  can  be  produced  on  a  small  region.  In  scattering 
the  spores  there  is  also  an  advantage  in  having  tli(»  spoi'ophylls 
at  the  top  of  the  stem. 

Gametophyte.  —  When  the  spores  fall  to  tlic  ground  and 
germinate,  they  develop  fleshy  gametophytes  consisting  usually 
of  a  tuberous  subterranean  portion  from  which  small,  aerial, 
green  lobes  arise  on  which  the  sex  organs  are  produced.     Within 


Fig.  393.  —  The  sporophyte  of  a  Selaginella.     After  J.  M.  Coulter. 

the  tissues  of  the  gametophyte  there  lives  a  filamentous  Fungus, 
and  thus  it  is  seen  that  the  gametophyte  resembles  the  gameto- 
phyte of  Botrychium  in  a  number  of  ways. 

The  fertilized  egg  begins  to  develop  immediately  after  fertiliza- 
tion, and  the  young  sporophyte  is  soon  formed  and  the  life  cycle 
thus  completed. 

Selaginella.  —  The  Selaginellas,  known  as  Little  Club  Mosses, 
are  widely  distributed  over  the  world  and  are  common  in  con- 
servatories where  they  are  grown  under  the  benches,  in  pots,  and 
in  hanging  baskets  for  their  decorative  effect. 


SPOROPHYTE 


441 


Sporophyte.  —  The  sporophytes  arc  delicate  plants  with  leafy 
much  branched  stems  {Fig.  393).  The  strobiU  occur  on  the  ends 
of  the  branches,  and  the  sporophylls  somewhat  resemble  the  foli- 
age leaves,  but  are  usually  smaller  and  more  compact  {Fig.  394). 

One  notable  feature  is  that  there  are  two  kinds  of  spores  pro- 
duced.    In   Bryophytes,   True   Ferns,    Horsetails,   and   Lycopo- 


FiG.  394.  —  The  vegetative  and  spore-bearing  structures  of  the  sporo- 
phyte of  Selaginella.  A,  a  shoot  of  Selaginella,  showing  the  stem,  vegetative 
leaves,  and  the  strobili  {st)  at  the  ends  of  the  branches  (X  2).  B,  a  micro- 
sporophyll,  showing  the  microsporangium  (in)  which  has  opened  to  allow  the 
microspores  to  escape  (X  about  10).  At  the  right  of  the  microsporophyll 
are  shown  two  microspores  (s)  (X  50) .  C,  megasporophyli  with  megasporan- 
gium  {me)  open,  thus  exposing  the  four  megaspores  and  permitting  the  micro- 
spores to  come  in  contact  with  the  megaspores.  Below  the  megasporophj'll 
are  shown  two  megaspores  {n)  (X  about  20).  D,  lengthwise  section  through 
a  portion  of  a  shoot,  showing  the  position  of  the  two  kinds  of  sporangia  in 
relation  to  the  leaves,  and  also  the  relative  sizes  of  the  two  kinds  of  spores 
(X  15).     Partly  from  Dodel-Port  and  partly  from  nature. 


442  PTERIDOPHYTES    (FERN   PLANTS) 

diums  the  spores  are  alike  as  to  size,  although  in  some  cases  they 
differ  in  the  kinds  of  ganietophytes  produced.  Other  Pterido- 
phytes  differentiated  spore-bearing  and  vegetative  tissues,  but 
the  Selaginellas  have  differentiated  spores  both  in  size  and  func- 
tion. The  larger  spores,  which  are  many  times  larger  than  the 
smaller  ones,  produce  only  female,  while  the  smaller  ones  produce 
only  male  ganietophytes.  The  two  kinds  of  spores  are  borne 
in  separate  sporangia  which  also  differ  in  size.  The  prefixes, 
micro,  meaning  little,  and  mega  or  macro,  meaning  large,  are  used 
to  designate  these  spores  and  also  the  sporangia  and  sporophylls 
which  bear  them.  Thus  we  speak  of  microspores  and  megaspores, 
microsporangia  and  megasporangia,  and  inicrosporophylls  and  mega- 
sporophylls  (B  and  C,  Fig.  394). 

This  habit  of  producing  two  kinds  of  spores  in  regard  to  size  is 
called  heterospory  (meaning  different  spores),  while  the  habit  of 
producing  spores  alike  in  size  is  called  hom,ospory  (meaning  same 
spores).  The  introduction  of  heterospory  by  Seiaginella  is  a 
significant  feature  because  all  Seed  Plants  are  heterosporous.  In 
Seed  Plants  the  pollen  grains  are  microspores  and  within  the 
ovules  occur  the  megaspores, 

Gametophytes.  —  The  second  notable  feature  which  Seiagi- 
nella presents  is  that  the  gametophytes  are  so  much  reduced  that 
they  develop  within  the  spores,  where  food  and  protection  are 
provided.  Thus  in  Seiaginella  there  are  no  green  independent 
gametophytes  as  we  have  been  used  to  in  other  Pteridophytes 
and  in  Bryophytes,  but  the  gametophyte  now  lives  on  the  sporo- 
phyte  just  as  the  sporophyte  of  the  Bryophytes  lives  on  the 
gametophyte.  This,  also,  is  a  feature  that  is  characteristic  of 
Seed  Plants. 

The  male  gametophyte  is  extremely  simple,  consisting  of  one 
vegetative  cell  and  a  simple  anthcridium  containing  only  a  few 
sperms,  each  of  which  nas  two  slender  ciUa  (C,  Fig.  395).  In 
developing,  the  male  gametophyte  breaks  the  spore  wall,  so  that 
a  crack  is  produced  through  which  the  sperms  escape. 

The  megaspores  germinate  and  form  the  female  gametophytes 
while  still  in  the  sporangium,  and  this  is  a  third  feature  that  is 
characteristic  of  Seed  Plants.  The  female  gametophyte  is  much 
larger  than  the  male  gametophyte  {A  and  B,  Fig.  395).  Its  much 
larger  size  is  permitted  by  the  greater  size  of  the  megaspore  and 
is  also  necessary  because  the  female  gametophyte  must  support 


GAMETOPHYTES 


443 


the  young  sporopliyte  antil  it  becomes  self-supporting.  The 
female  gametopliyte  tlierefore  consists  of  many  cells  when  mature 
and  bears  a  number  of  archegonia  on  the  portion  exposed  by  the 
opening  forced  in  the  spore  wall  by  the  expansion  of  the  game- 
tophyte. 

Previous  to  fertilization,  the  male  gametophytes,  each  still, 
except  for  a  small  sht-like  opening,  encased  in  the  wall  of  the 


Fig.  395.  —  The  gametophytes  and  young  sporophyte  of  Selaginella.  A, 
a  megaspore  containing  a  female  gametophyte  with  the  portion  bearing  the 
archegonia  exposed  by  the  sUt-like  opening  in  the  spore  wall  (X  100).  B, 
section  through  a  megaspore,  showing  the  spore  wall  (w)  and  female  game- 
tophyte (gr)  with  one  archegonium  (a)  with  neck  and  egg  (e)  visible  (X  100). 
C,  a  section  through  an  antheridium,  showing  the  small  prothallial  cell  at  the 
base  and  the  wall  cells  which  enclose  the  sperms  withui,  one  of  which  is  shown 
fully  mature  at  the  left  (X  500).  D,  a  young  sporophyte  with  stem  at  s  and 
root  at  r  and  foot  extending  into  the  gametophyte  which  is  still  enclosed  in 
the  spore  wall  (///).     From  Atkinson  and  nature. 

microspore,  fall  out  or  are  blown  out  of  the  microsporangia,  which 
open  when  the  spores  are  mature,  and  fall  or  are  carried  by  the 
wind  to  the  megasporangia  where  the  female  gametophytes  are 
developing.  Here  the  sperms  escape,  and  reach  the  archegonia, 
which  are  accessible  through  the  sht-like  openings  in  the  walls  of 
the  megasporangia  and  megaspores.  The  fertilized  egg  develops 
immediately  into  a  sporophyte.  Often  the  female  gametophyte 
remains  in  the  megasporangium  until  the  weight  of  the  young 
sfiorophyte  tumbles  it  out.     After  the  young  sporophyte  becomes 


444  PTERIDOPHYTES   (FERN   PLANTS) 

established  in  tlie  soil  and  readies  maturity,  strobili  are  produced 
and  thus  the  life  cycle  is  completed.  Thus  besides  having  an 
independent  complex  sporophyte,  the  Selaginellas  protect  their 
gametophytes  and  this  is  an  additional  adjustment  to  the  land 
habit. 

Summary  of  Pteridophytes.  —  They  present  a  number  of 
features  characteristic  of  Seed  Plants.  They  have  an  inde- 
pendent sporophyte  with  well  developed  roots,  stems,  and  leaves, 
which  in  general  have  the  same  tissues  that  are  characteristic 
of  these  organs  in  Seed  Plants.  The  second  important  feature 
is  the  differentiation  of  vegetative  and  spore-bearing  tissues. 
This  gave  rise  to  the  strobilus  which  is  regarded  as  the  forerunner 
of  the  flower.  The  third  important  feature  is  the  introduction 
of  heterospory  and  the  production  of  gametophytes  within  the 
spore  wall.  Heterospory  and  dependent  gametophytes  made  the 
origin  of  the  seed  possible.  The  fourth  feature  is  the  retention 
of  the  female  gametophyte  within  the  megasporangium  during 
fertilization.     This  also  is  a  seed-like  feature. 


CHAPTER   XVIII 

SPERMATOPHYTES    (SEED   PLANTS) 

Gymnosperms  (seed  not  enclosed) 

Spermatophytes  or  Seed  Plants  constitute  the  fourth  large 
division  of  plants.  They  are  the  most  highly  developed  plants, 
and,  therefore,  in  them  we  find  the  final  achievement  of  plant 
evolution.  Their  distinguishing  feature  is  the  seed,  although 
they  have  other  notable  features  not  found  in  the  groups  pre- 
viously studied.  The  notable  features  of  Pteridophytes,  such  as 
sporophylls,  strobili,  heterospory,  dependent  gametophytes  that 
are  developed  within  the  spore  wall,  and  the  retention  of  the 
megaspore  in  the  megasporangium,  are  retained  by  the  Sperma- 
tophytes and  to  these  they  have  added  new  features.  Because 
of  the  seed,  lumber,  fibers,  and  numerous  other  products  ob- 
tained from  them,  the  Spermatophytes  surpass  all  other  divisions 
of  plants  in  economic  importance.  They  are  also  very  numerous, 
and  on  account  of  the  large  size  of  their  sporophytes  they  are  our 
most  conspicuous  plants. 

The  Spermatophytes  are  divided  into  two  groups,  —  the  Gijm- 
nosperms  and  Angiosperms.  As  the  names  suggest,  the  Gym- 
nosperms bear  their  seeds  exposed  while  Angiosperms  bear  them 
enclosed,  but  the  two  groups  differ  also  in  other  features  as  will 
be  noted  later. 

The  Gymnosperms  are  more  primitive  than  the  Angiosperms 
and  are,  therefore,  more  hke  the  Pteridophytes,  the  group  from 
which  Seed  Plants  are  supposed  to  have  originated.  The  groups 
of  Gymnosperms  most  like  Pteridophytes  are  now  extinct  and 
hence  are  known  only  by  their  fossils.  Some  of  these  extinct 
forms  resembled  Ferns  so  much  that  they  are  called  Pterido- 
sperms,  a  term  which  means  "  Ferns  with  seeds."  Thus  Gymno- 
sperms connect  more  closely  with  the  Pteridophytes  than  the 
latter  group  does  with  the  Bryophytes.  The  Gymnosperms  still 
in  existence  are  divided  into  a  number  of  groups,  but  a  study  of 
the  Cycads  and  Pines  will  give  a  notion  of  the  general  features 
characteristic  of  Gymnosperms. 

445 


446 


SPERMATOPHYTES    (SEED   PLANTS) 


Cycads 

Of  the  Gymiiospcrms  now  in  existence,  the  Cycads  bear  most 
resemblance  to  the  Ferns.  In  leaf  and  stem  characters,  some  of 
them  could  easily  be  mistaken  for  Ferns  {Fig.  396).  There  are 
nearly  one  hundred  species  of  Cycads.  They  are  tropical  plants 
but  are  grown  nearly  everywhere  in  greenhouses.     One  of  the 


Fig.  396.  —  A  Cycad,  showing  the  finely  divided  leaves  and  the  short  thick 
trunk  with  its  rough  covering  of  leaf  bases.     After  J.  M.  Coulter. 


forms  (Cycas  revoluta)  common  in  cultivation  is  often  labeled 
"  Sago  Palm  "  because  its  leaves  resemble  those  of  some  of  the 
Palms. 

Sporophyte.  —  The  sporophyte  has  a  tuberous  or  columnar 
stem  at  the  top  of  which  are  borne  the  large,  much  branched, 
fern-like  leaves.  The  stems  are  covered  by  the  leaf-bases  which 
remain  after  the  leaves  fall.  In  some  Cycads,  where  the  stem  is 
subterranean,  the  plant  is  small,  but  in  others  with  columnar 
stems,  the  plant  may  reach  a  height  of  50  feet  or  more. 

Strobili.  —  The  strobili  are  borne  near  the  apex  of  the  stem  of 
which  they  are  really  branches,  and  are  of  two  kinds  —  staminate 
and  ovulate.  The  staminate  strobili  are  simply  microstrobili, 
that  is,  strobili  in  which  only  microsporophylls  and  microspo- 


STROBILI 


447 


rangia  are  produced.  The  name,  however,  suggests  the  likeness 
of  the  microsporophylls  to  the  stamens  of  Flowering  Plants. 
The  ovulate  strobili  are  strobili  in  which  only  megasporophylls 
and  megasporangia  occur.  The  term  ovulate  suggests  the  like- 
ness of  the  megasporangium  to  the  ovule  of  Flowering  Plants. 
The  megasporangia  are  now  called  ovules  because  they  remain 
closed,  so  that  the  female  gametophyte  is  at  no  time  exposed. 

It  is  obvious  that  the  Cycads  have  carried  the  differentiation 
of  structures  farther  than  the  Selaginellas  have.     In  Cycads,  not 


Fig.  397.  — •  Staminate  strobilus  and  microsporophylls  in  Cycads.  At  the 
left,  a  staminate  strobilus  of  a  Cycad  {Dioon);  at  the  right,  microsporo- 
phylls from  two  different  Cycads,  showing  difference  in  shape,  and  the  way 
the  sporangia  are  borne.     After  Chamberlain  and  Richard. 


only  spores,  sporangia,  and  sporophylls  are  differentiated,  but 
there  is  also  a  differentiation  of  strobili. 

The  strobili  of  Cycads  are  much  larger  than  those  of  Selaginella 
or  Lycopodium,  and  the  sporophylls  are  usually  very  different 
from  the  foliage  leaves.  In  some  Cycads  the  strobili  are  a  foot  or 
more  in  length  and  several  inches  in  diameter. 

In  the  staminate  strohili,  the  sporophylls  are  closely  crowded 
and  practically  have  no  resemblance  to  foliage  leaves.  They 
vary  considerably  in  shape  in  different  Cycads,  but  have  an 
outer,  expanded,  sterile  portion  and  bear  the  microsporangia, 
usually  grouped  in  sori,  on  their  under  surface  {Fig.  397). 

The  ovulate  strobili  are  often  much  larger  than  the  staminate 
strobili.     The  megasporophylls  are  usually  closely  crowded,  and 


448 


SPERMATOPHYTES    (SEED   PLANTS) 


when  they  are  sliort  and  ficshy,  they  fit  togetlier  like  the  kernels 
on  an  ear  of  Corn,  The  ovules  are  borne  separately  near  the 
base  of  the  niegasporophyll  and  as  shown  in  Figure  398.  In  some 
Cycads  each  niegasporophyll  bears  only  two  ovules,  while  in 
others,  as  Figure  398  shows,  a  larger  number  may  be  present.  In 
some  Cycads  the  megasporophylls  are  much  branched  like  foliage 
leaves,  and  the  sporangia  appear  to  be  transformed  lower  branches 
or  pinnae.  Megasporophylls  of  this  type  suggest  the  relation- 
ship of  sporophylls  to  foliage  leaves. 

The  young  megasporangium  or  ovule  contains  four  megaspores, 
which  are  enclosed  by  two  distinct  coverings  of  sterile  tissue. 
The  inner  covering  is  the  nucellus,  which  surrounds  and  encloses 


Fig.  398.  —  Ovulate  strobilus  and  megasporophylls  in  Cycads.  At  the 
left,  an  ovulate  strobilus;  at  the  right,  two  types  of  megasporophylls,  show- 
ing the  ovules  (o). 


the  megaspores,  and  the  outer  one  is  the  integument,  which  grows 
up  from  the  base  of  the  ovule  and  forms  a  covering  over  the 
nucellus.  The  integument  is  a  protection  for  the  nucellus,  and, 
when  the  ovule  develops  into  a  seed,  it  is  transformed  into  a  seed 
coat.  At  the  outer  end  of  the  megasporangium  where  the  integu- 
ment closes  over  the  nucellus,  a  small  opening  or  micropyle  is 
left  which  leads  into  a  cavity,  called  the  pollen  chamber,  into 
which  a  beak-like  portion  of  the  nucellus  projects  {Fig.  399). 

Female  Gametophyte.  —  Only  one  of  the  four  megaspores  in 
the  megasporangium  develops.  The  other  three  disappear  and 
all  of  the  space  and  food  is  therefore  given  over  to  the  develop- 
ment of  one  gametophyte.     The  megaspore  germinates  in  the 


FEMALE  GAMETOPHYTE 


449 


sporangium  as  in  Selaginella,  but  a  new  feature  of  the  Cyeads  is 
that  the  megasporangiuni  does  not  open  to  allow  the  megaspore 
to  be  exposed,  and  therefore  the  female  gametophyte  remains 
permanently  enclosed  in  the  sporangium.  The  developing  female 
gametophyte  uses  most  of  the  nucellus  for  food  and  thereby 
makes  room  for  itself.     When  the  gametophyte  is  mature  the 


Fig.  399.  —  Section  through  a  Cycad  ovule  containing  a  mature  gameto- 
phyte. /,  female  gametophyte  with  two  archegonia  (a)  shown;  m,  micro- 
spores developing  tubes,  and  male  gametophytes;  n,  nucellus;  i,  integument; 
p,  pollen  chamber  into  which  the  micropyle  shown  just  above  opens.  Re- 
drawn from  Webber. 


nucellus  is  so  nearly  used  up  that  it  is  reduced  to  a  thin  layer, 
except  at  the  micropylar  end  where  a  beak-like  portion  remains. 
A  female  gametophyte  when  fully  formed  consists  of  a  large  num- 
ber of  cells,  most  of  which  form  a  nutritive  tissue  for  the  devel- 
oping sporophyte  and  are  therefore  spoken  of  as  endosperm, 
although  the  endosperm  of  Cyeads  is  not  the  same  in  origin  as  the 
endosperm  of  Angiosperms.     The  archegonia,  usually  several  in 


450  SPERMATOPHYTES   (SEED    PLANTS) 

number,  are  produced  at  the  uiicropylar  end,  and  have  much 
shorter  necks  and  are  simpler  in  other  ways  than  the  archegonia 
of  Pteridophytes.  The  eggs  are  large  and  the  most  conspicuous 
part  of  the  archegonia.  A  section  through  an  ovule  ready  for 
fertilization  looks  like  the  one  shown  in  Figure  399. 

Male  Gametophyte.  —  The  microspores  or  pollen  grains,  as  they 
may  now  1)0  called  since  they  have  to  be  transferred  to  the  ovule 
before  they  can  function,  usually  contain  three-celled  gameto- 
phytes  at  the  time  of  their  shedding,  and  in  this  condition  they 
reach  the  megasporangium,  pass  through  the  micropyle,  and  reach 
the  pollen  chamber,  where  they  are  in  contact  with  the  beak  of 
the  nucellus.  In  this  position  the  three-celled  gametophyte, 
which  consists  of  a  vegetative,  generative,  and  tube  cell,  com- 
pletes its  development.  The  miscrospore  develops  tubes  which 
branch  and  penetrate  the  beak  of  the  nucellus  in  various  direc- 
tions, and  function  as  absorptive  structures.  Finally,  the  beak 
of  the  nucellus  breaks  down  and  thereby  a  passage  way  to 
the  archegonia  is  provided.  Meanwhile  the  generative  cell 
enters  one  of  the  pollen  tubes  and  passes  farther  into  the  pollen 
chamber  where  it  divides,  forming  a  stalk  cell  and  a  body  cell, 
the  latter  of  which  forms  the  sperms,  usually  two  in  number. 
The  sperms  bear  a  large  number  of  cilia,  and  after  escaping  from 
the  pollen  tube  they  swim  through  the  watery  solution  present 
in  the  chamber  and  thereby  reach  the  archegonia  and  finally  the 
eggs. 

Thus,  when  the  male  gametophyte  is  mature,  it  consists  of  only 
four  -cells  besides  the  sperms,  and  there  is  no  structure  formed  that 
resembles  an  antheridium.  In  addition  to  the  absence  of  an 
antheridium,  it  should  also  be  noted  that  pollination  and  the 
growth  of  tubes  are  other  new  features  which  occur  in  connection 
with  the  male  gametophytes  of  Cycads.  It  is  obvious  that  the 
introduction  of  pollination  and  the  growth  of  pollen  tubes  must 
accompany  the  permanent  enclosing  of  the  female  gametophyte 
in  the  megasporangium. 

Seed.  —  The  seed  is  another  new  feature  of  the  Cycads.  After 
fertilization,  a  young  sporophyte  {embryo)  is  developed  and  is 
pushed  well  down  into  the  nutritive  tissue  of  the  gametophyte 
by  a  filament  of  celh  (suspensor).  During  fertilization  and  the 
development  of  the  embryo,  the  ovule  continues  to  grow  and 
the  integument  becomes  pulpy,  while  the  outer  region  of  the  re- 


PINES    (PINACEAE) 


451 


maining  portion  of  the  iiuccllus  hardens,  so  that  the  seed  when 
mature  resembles  some  of  the  stone  fruits,  such  as  the  Plum, 
although  it  is  a  seed  and  not  a  fruit. 

It  is  obvious  that  a  seed  is  simply  a  transformed  megaspo- 
rangium.     In  the  Cycads  a  seed  is  a  megasporangium  which  has 
its  outer  portions  modified .  for  protection  and  contains  within 
a   female   gametophyte   bearing   a 
young  sporophyte.      Thus  the  re- 
duction of  the  female  gametophyte 
through    the     Pteridophytes     and 
finally  its  retention  in  the  mega- 
sporangium in  the  Cycads  so  that 
the  young  sporophyte  also  develops 
within   the   megasporangium  were 
important  steps  in  the  evolution  of 
the  seed. 

''^•'Although  the  Cycads  resemble 
Ferns  in  having  swimming  sperms, 
and  in  having  leaves  and  stems  that 
are  Fern-like,  they  contrast  with 
them  in  such  new  features  as  differ- 
entiation of  strobili,  simpler  ga- 
metophytes,  pollination,  growth  of 
pollen  tuljes,  and  the  seed. 

Pines    (Pinaceae) 

The  Pines  are  a  subdivision  of  the 
Pine  family  (Pinaceae) .  In  addition 
to  the  Pines,  the  Pine  family  in- 
cludes the  Spruces,  Firs,  Hemlocks, 
Larches,  Cedars,  Redwood,  Cypre>ss, 
and  others.  The  Pine  family  is  an 
exceedingly  important  one  because  it  includes  a  large  proportion 
of  the  trees  from  which  lumber  is  obtained.  The  Pine  family 
belongs  to  the  order  of  Conifers  (Comfemles),  so  named  because 
of  the  cones  which  they  bear.  Not  all  of  them,  however,  bear 
dry  cones  like  the  Pines,  for  some  have  fleshy  fruit-like  structures, 
as  the  berry-like  structures  of  the  Junipers  illustrate.  All  of  the 
representatives  of  the  Pine  family  are  interesting,  but  a  study  of 
their  life  history  will  be  limited  to  that  of  the  Pine. 


Fig.  400.  —  Pine  sporophytes. 
After  Miss  Hayden. 


452 


SPERMATOPHYTES   (SEED   PLANTS) 


Sporophyte.  —  The  sporophytes  of  the  Pines  are  mostly  large 
and  in  some  eases  are  of  huge  dimensions.  Some  species  of  Pine 
attain  a  height  of  150  feet  or  more.  It  is  characteristic  of  Pine 
trees  to  have  a  main  trunk  and  comparatively  small  lateral 
branches.  The  main  branches  are  usually  in  clusters,  and  in 
some  Pines,  unless  closely  inspected,  one  might  mistake  the 
branches  to  be  in  whorls.     There  is  a  gradual  reduction  in  length 

of  branches  from  below  up- 
ward, so  that  trees  grown 
in  the  open  have  a  conical 
shape  {Fig.  4OO.) 

The  needle-like  leaves 
are  usually  borne  in  groups 
or  fascicles  of  two,  three, 
or  five  leaves  according  to 
the  species.  The  duration 
of  leaves  varies  according 
to  the  species  and  condi- 
tions, but  Pines  shed  only 
a  part  of  their  leaves  at  a 
time  and  hence  are  always 
green. 

Strobili.  —  The  strobili, 
as  in  the  Cycads,  are  of 
two  kinds  —  staminate  and 
ovulate  {Fig.  4OI).  The 
staminate  and  ovulate 
strobili  occur  separately, 
on  the  same  trees,  or  on 
different  trees. 

The  staminate  strobili  or 
cones  {Fig.  Jt.02)  are  pro- 
duced in  clusters  and  in  the  Northern  states  may  be  seen  in  May  or 
early  June.  They  vary  in  size  in  different  species,  sometimes  at- 
taining a  length  of  half  an  inch  or  more,  but  in  many  species  they 
are  much  smaller.  They  expand  from  the  buds  in  a  few  days, 
soon  shed  their  pollen  and  disappear,  usually  persisting  only  a  few 
weeks.  A  microstrobilus  is,  in  reality,  a  modified  branch  con- 
sisting of  a  main  axis  bearing  scale-like  microsporophylls  or 
gtamens,  which  are  arranged  spirally  and  closely  crowded.     On 


Fig.  401.  —  A  branch  of  a  Pine,  show- 
ing an  ovulate  strobilus  at  a  and  a  cluster 
of  staminate  strobili  at  h. 


STROBILI 


453 


the  back  or  lower  side  of  the  niicrosporophylls  are  the  micro- 
sporangia,  usually  two,  and  each  contains  numerous  microspores. 
Nearly  opposite  each  other  on  the  microspore  are  two  air-sacs 
whereby  the  spores  are  easily  carried  by  the  wind.  When  the 
spores  are  mature,  the  microsporangia  or  pollen  sacs  open  by 
longitudinal  slits,  and  the  pollen  shatters  out,  often  like  small 


Fig.  402.  The  staminate  structures  of  the  Pine.  A,  cluster  of  staminate 
strobiU  ( X  about  f ) .  B,  a.  staminate  strobilus  enlarged,  showing  the  arrange- 
ment of  the  microsporophylls.  C,  a  microsporophyll,  showing  the  two 
sporangia  (m) ;  D,  microspore  showing  the  two  wings  and  two  cells  of  the  male 
gametophyte. 


clouds  of  dust.  The  wind  carries  the  pollen  about,  and  some 
reaches  the  ovulate  strobili,  but  much  the  larger  part  of  it  is 
wasted.  Sometimes  pollen  accumulates  on  walks  under  Pines 
that  are  shedding  their  pollen  until  the  walks  look  as  if  they  had 
been  sprinkled  with  finely  powdered  sulphur. 

The  ovulate  strohili  or  cones  appear  near  the  tips  of  the  new 
growths  in  early  spring.  Usually  they  are  smaller  when  they 
first  appear  than  the  staminate  cones,  but  they  persist  and,  after 
a  growth  of  two  seasons,  become  the  conspicuous  scaly  cones  so 
familiar  on  or  about  pine  trees.     Sometimes  several  occur  to- 


454 


SPERMAT0PHYTE8    (SEED  PLANTS) 


gether,  but  they  do  not  form  close  clusters  as  the  staminate 
cones  do. 

The  scales  of  the  ovulate  cones  are  considered  too  complex  to 
be  called  sporoph^dls,  for  each  scale  consists  of  an  ovuliferous 
scale  (ovulo-bearing  scale)  and  a  bract,  the  two  being  partly  united. 
Some  morphologists  think  that  the  ovuliferous  scale  itself  repre- 
sents two  sporophylls  fused  together.     The  megasporangia  or 


Fig.  403.  —  The  ovulate  structures  of  the  Pine.  A,  branch  bearing  four 
ovulate  strobiH,  B,  ovulate  strobilus,  showing  the  arrangement  of  scales 
( X  about  2) ;  C,  a  view  of  the  inner  or  upper  side  of  a  scale,  showing  the  two 
sporangia  (s). 


ovules,  two  in  number,  are  borne  on  the  upper  side  and  at  the 
base  of  the  ovuliferous  scale  {Fig.  403).  The  scales  are  spirally 
arranged  and  closely  crowded,  but  during  pollination  they  spread 
apart,  and  the  pollen  can  slide  in  between  them  and  reach  the 
ovules.  After  pollination  the  scales  close  together  again,  and  the 
cone  is  made  water-tight  by  a  secretion  of  resin.  After  pollina- 
tion the  cone  also  changes  from  the  vertical  to  the  nodding 
position. 

The  ovules  consist  of  an  integument  and  a  nucellus,  and  deeply 
buried  within  the  nucellus  the  four  megaspores  occur.  The 
ovules  are  arranged  one  oh  each  side  of  the  median  line  of  the 
scale,  with  the  micropyles  pointing  downward.     The  integument 


FEMALE   GAMETOPHYTE 


455 


extends  beyond  the  nucellus,  and  its  free  niai'gin  flares  open,  thus 
forniin<^  an  open  inic.ropyle  that  leads  into  the  pollen  chamber. 

Female  Gametophyte. — ^  Although  four  niegaspores  are  formed 
in  the  megasporangium,  only  one  of  them  develops  a  gameto- 
phyte, the  others  being  destroyed  and  used  for  food  by  the  one 
that  develops.  During  the  first  season  the  surviving  megaspore 
enlarges  and  becomes  multinucleate.     With  the  megaspore  in  this 


Fig.  404.  —  Development  of  the  ovule  and  pollen  tubes  in  the  Pine.  C, 
section  through  an  ovuliferous  scale,  showing  the  bract  behind  and  a  section 
of  an  ovule  (s)  on  its  inner  face,  the  megaspore  being  shown  at  m;  D,  an 
ovule  with  female  gametophyte  (/)  mature,  showing  eggs  at  e,  ovule  wall 
consisting  of  nucellus  and  integument  at  w,  and  pollen  grains  growing  tubes 
through  the  nucellus  at  (p). 


condition  the  ovule  passes  the  winter.  Early  next  spring  growth 
is  resumed,  and  by  about  the  first  of  June  of  the  second  season 
the  gametophyte  is  complete,  consisting  of  250  or  more  cells  and 
bearing  a  number  of  archegonia  (usually  two  to  five)  at  the 
micropylar  end  {Fiy.  404-)  The  eggs  are  usually  ready  for 
fertilization  about  the  first  of  June  of  the  second  season.  While 
the  female  gametophyte  is  developing,  the  male  gametophyte  is 
completing  its  development  in  the  pollen  chamber  and  the  pollen 


456 


SPERMATOPHYTES   (SEED  PLANTS) 


tube   is   eating   its   way   through   the   nuceUus   to   the  female 
gametophyte. 

Male  Gametophyte.  —  The  male  gametophyte  forms  within 
the  pollen  grain  and  its  tube.  At  the  time  of  pollination  the  male 
gametophyte  commonly  consists  of  four  cells  —  two  prothallial 
or  vegetative  cells,  a  generative  cell,  and  a  tube  cell.  At  least  one 
of  the  prothallial  cells  usually  disintegrates  and  disappears  early 
in  the  development  of  the  gametophyte.  This  is  the  condition 
of  the  male  gametophyte  when  the  pollen  is  carried  to  the  ovulate 
cone.  Upon  reaching  the  ovulate  cones  the  pollen  grains  fall 
down  to  the  base  of  the  scales  in  the  region  of  the  ovules,  and 


Fig.  405.  —  Seed  structures  of  the  Pine.  A,  a  mature  ovulate  strobilus 
with  scales  spread  apart  to  allow  the  seeds  to  escape.  B,  a  view  of  the  inner 
side  of  a  scale,  showing  the  two  seeds  when  mature.  The  wings  of  the  seeds 
are  a  part  of  the  scale  and  did  not  develop  from  the  ovule.  C,  section  through 
a  pine  seed,  showing  the  female  gametophyte  {g),  embryo  (e),  and  seed  coat  (w) . 


some  lodge  at  the  mouth  of  the  micropyles,  where  they  are  caught 
in  a  drop  of  a  mucilaginous  secretion  and  drawn  in  close  to  the 
tip  of  the  nucellus.  In  this  position  the  pollen  grains  begin  to 
develop  tubes,  which  by  means  of  an  enzyme  dissolve  the  nucel- 
lar  tissue,  using  it  as  food  and  at  the  same  time  making  a  way  for 
themselves.  Cold  weather  finally  checks  the  growth  of  the 
pollen  tubes,  and  the  male  gametophytes  now  rest  over  winter. 
Early  the  next  spring  the  pollen  tube  resumes  its  growth  toward 
the  archegonia,  and  the  generative  cell  passes  into  the  pollen  tube 
and  divides,  forming  two  cells,  one  of  which  divides  and  forms  the 
two  sperms  whicli  now  have  the  pollen  tube  as  a  passageway  to 
the  archegonia.     The  sperms  reach  the  archegonia  about  the 


SEED 


457 


middle  of  June  of  the  second  season  and  fertilization  soon  follows. 
In  addition  to  its  simplicity  the  notable  features  of  the  male 
gametophytc  are  that  the  sperms  have  no  cilia  and  that  they  are 
conducted  to  the  archegonia  by  the  pollen  tube. 

Seed.  —  The  fertilized  egg  at  first  forms  tiers  of  cells,  which 
constitute  a  long  filament,  called  a  suspensor,  at  the  end  of  which 
the  embryo  develops  deeply  imbedded  in  the  nutritive  tissue  of 
the  female  gametophyte.  When  mature  the  embryo  is  still 
surrounded  by  much  gametophytic  tissue  called  endosperm. 

While  the  embryo  or  the  young  sporophyte  is  developing,  the 
ovule  and  the  entire  cone  continue  to  enlarge.  The  integument 
is  transformed  into  a  seed  coat,  and  when  mature  the  seed  sepa- 


Ir 


rce 


ree 


Fig.  406.  —  Diagram  of  the  life  cycle  of  the  Pine.  Starting  with  the  tree 
at  the  left,  the  two  kinds  of  strobili  are  shown  at  a  and  b,  the  two  kinds  of 
sporophylls  and  their  sporangia  at  c  and  d,  the  two  kinds  of  spores  at  e  and  /, 
the  gametophytes  at  g,  the  mature  seed  at  h,  from  the  embryo  of  which  a 
new  tree  develops. 


rates  from  the  ovulate  scale  with  a  long  membraneous  wing, 
which  enables  the  seed  to  float  in  the  air  {Fig.  405.)  Pine 
seeds,  although  usually  smaller,  are  similar  in  general  structure 
to  the  seeds  of  Cycads.  They  contain  a  female  gametophyte 
bearing  a  young  sporophyte  and  a  protective  covering  composed 
of  the  integument  and  the  nucellus,  the  latter  persisting  as  a 
membrane  about  the  gametophyte  or  endosperm. 

The  scales  of  the  ovulate  strobilus  continue  their  development 
until  the  seeds  are  mature  and  remain  tightly  closed  so  that  the 
seeds  are  well  protected.  «■  After  the  cone  is  mature,  the  scales  dry 


458  SPERMAT0PHYTE8    (SEED   PLANTS) 

and  spread  apart  and  the  seeds  fall  out.  Although  the  seeds  are 
protected  between  the  scales,  they  are  not  enclosed  as  the  seeds  of 
a  Bean  or  an  Apple  are.  They  are  on  the  outside  of  the  structure 
which  bears  them,  —  whence  the  name  Gymnosperms. 

The  seeds  are  dispersed  by  the  wind  and  usually  tlo  not  germi- 
nate until  the  next  spring  after  dispersal.  In  germination  the 
axis  (hypocotyl)  of  the  sporophyte  elongates,  forming  an  arch 
and  drawing  the  cotyledons  out  of  the  ground,  and  at  the  same 
time  the  tap-root  at  the  lower  end  of  the  hypocotyl  becomes 
established  in  the  soil.  By  the  straightening  of  the  hypocotyl 
the  green  cotyledons  are  lifted  into  the  air  and  sunlight,  and  the 
sporophyte  soon  becomes  independent  of  the  seed.  After  a 
number  of  years  of  growth,  it  begins  to  bear  strobili,  thus  com- 
pleting the  life  cycle  of  the  Pine  as  shown  in  Figure  406. 

In  summarizing  it  should  be  noted  that  the  Pines  have  two 
kinds  of  strobili,  reduced  gametophytes,  pollination,  and  pollen 
tubes,  features  which  were  pointed  out  as  the  notable  ones  of  the 
Cycads.  But  in  contrast  with  the  Cycads  the  Pines  have  more 
massive  sporophytes  with  leaves  bearing  no  resemblance  to  those 
of  Ferns,  and  also  the  Pines  have  abandoned  swimming  sperms 
and  conduct  the  sperms  to  the  eggs  through  pollen  tubes. 

In  pines  the  cones  mature  the  second  fall  after  pollination,  but 
in  some  genera  of  the  pine  family,  as  the  Spruces  illustrate,  sexual 
reproduction  proceeds  more  rapidly,  although  similar  in  nature, 
and  the  cones  mature  the  fall  following  polUnation. 


CHAPTER   XTX 

SPERMATOPHYTES    (Continued) 

Angiosperms  (Seeds  Enclosed) 

General  Characteristics.  —  The  Angiosperms  are  the  most 
highly  evolved  group  of  the  plant  kingdom,  being  the  most  per- 
fectly adapted  to  terrestrial  conditions.  They  also  surpass  all 
other  groups  in  economic  importance,  for  they  include  the  large 
majority  of  our  cultivated  plants.  Our  dependence  upon  the 
grains  and  fruits  and  upon  forage,  root,  and  tuber  crops  attests 
the  economic  importance  of  the  Angiosperms.  The  Angiosperms 
probably  have  more  species  than  any  other  group  of  plants  and 
show  more  variations.  Approximately  125,000  species  are 
known.  They  form  the  most  conspicuous  part  of  our  vegetation, 
for  not  only  most  of  our  cultivated  plants  but  nearly  all  weeds 
are  Angiosperms.  The  origin  of  the  Angiosperms  is  not  known, 
but  they  probably  arose  from  some  Fern-like  plants  as  the  Gym- 
nosperms  did.  The  Angiosperms,  as  the  name  suggests,  are 
characterized  by  having  their  seeds  enclosed.  The  enclosure  is 
the  ovary,  which  is  one  of  the  notable  features  of  Angiosperms. 
Another  notable  feature  is  the  flower,  which  is  regarded  as  a 
special  type  of  strobilus.  They  also  differ  from  the  Gymno- 
sperms  in  having  more  reduced  gametophytes.  Both  male  and 
female  gametophytes  consist  of  only  a  few  cells  and  have  lost  all 
traces  of  sex  organs.  Since  the  gametophytes  are  microscopical, 
most  people  are  acquainted  with  only  the  sporophytes  of  Angio- 
sperms. In  character  of  roots,  stems,  leaves,  flowers,  seeds,  and 
fruits,  there  are  numerous  variations  in  Angiosperms,  but,  since 
Part  I  of  this  book  is  devoted  chiefly  to  these  variations,  the  dis- 
cussion will  now  be  limited  to  the  characteristic  features  of  the 
group  and  to  such  features  as  characterize  the  families  of  most 
economic  importance. 

The  Flower.  — -The  flower  (Fig.  407),  consisting  of  a  perianth 
(calyx  and  corolla),  stamens,  and  one  or  more  pistils,  is  a  structure 

459 


460 


SPERMATOPHYTES   (SEED  PLANTS) 


characteristic  of  Angiosperms.  The  stamens  are  microsporo- 
phylls  and  the  pistils  are  megasporophylls.  A  typical  flower  is, 
therefore,  essentially  an_  association  of  sporophylls  surrounded 
by  a  perianth,  and,  in  so  far  as  a  flower  is  an  association  of  sporo- 
phylls, it  does  not  differ 
fundamentally  from  a  stro- 
bilus.  In  passing  from  the 
simplest  Angiosperms, 
where  there  are  flowers 
that  have  no  perianth,  to 
those  Angiosperms  having 
typical  flowers,  all  grada- 
tions between  a  typical 
strobilis  and  a  typical 
flower  can  be  found.  It  is, 
therefore,  impossible  to  de- 
fine a  flower  so  as  to  in- 
clude the  flowers  of  all 
Angiosperms  and  at  the 
same  time  separate  the 
flower  from  the  strobilus. 
The  flowers  of  Angiosperms 
and  the  strobili  of  the  Gym- 
nosperms  and  P  t  e  r  i  d  o- 
phytes  differ  in  the  char- 
acter of  their  sporophylls 
more  than  in  any  other 
feature. 

Perianth.  —  The  peri- 
anth, usually  consisting  of 
both  sepals  and  petals,  not 
only  protects  the  sporo- 
phylls during  their  develop- 
ment but  also  serves  in 
pollination,  which  in  Angiosperms  is  done  largely  by  insects. 
At  the  base  of  the  perianth  occur  nectar  glands,  which  are  further 
adaptations  to  insect  pollination.  The  perianth  seems  to  have 
arisen  in  two  ways.  In  some  cases  there  is  evidence  that  the 
parts  of  the  perianth  are  modified  sporophylls,  while  in  other 
cases  they  are  apparently  modified  foliage  leaves. 


Fig.  407.  —  The  floral  structures  of  a 
typical  flower.  The  floral  structures  com- 
prise a  perianth  (a)  composed  of  calyx 
and  corolla,  a  number  of  microsporophylls 
or  stamens  each  consisting  of  anther  (c) 
and  filament  (c),  and  a  pistil  (b)  composed 
of  one  or  more  megasporophylls  with  the 
megasporangia  or  ovules  (d)  enclosed  in  an 
ovary. 


STAMEN 


461 


Stamen.  —  The  stamen  (microsporophyll)  has  its  pollen  sacs 
(microsponingia),  usually  four  in  number,  joined  into  the  struc- 
ture called  anther.  The  pollen  grains  (microspores)  are  numer- 
ous in  each  sac  and  are  formed  before  the  flower  opens:  Like 
the  spores  of  Gymnosperms,  Pteridophytes,  and  Bryophytes, 
they  are  formed  by  special  cells  known  as  mother  cells  of  which 
there  are  many  in  each  pollen  sac  as  shown  at  A  in  Figure  408. 
These  mother  cells  also  divide  by  the  reduction  division,  that  is, 
by  the  kind  of  cell  division  in  which  the  daughter  nuclei  get  only 
half  the  sporophytic  number  of  chromosomes.     The  mother  cells 


A  B 

Fig.  408.  —  The  spore  mother  cells  of  Angiosperms.  A,  cross  section  of 
a  young  anther,  showing  the  microspore  mother  cells  (//i).  B,  section  through 
an  ovule,  showing  the  megaspore  mother  cell  (?»).     Both  are  highly  magnified. 


are  formed  and  undergo  the  reduction  division  while  the  flowers 
are  still  small  buds.  Immediately  following  the  division  of  the 
mother  cell,  the  daughter  nuclei  resulting  from  this  division 
divide  and  consequently  there  are  four  spores  or  pollen  grains 
formed  from  each  mother  cell.  The  four  spores  constituting 
the  progeny  of  a  mother  cell  are  called  a  tetrad.  The  cells  of  the 
tetrad  commonly  cling  together  for  a  short  time  after  they  are 
formed,  but  soon  separate  and  each  becomes  a  pollen  grain.  The 
pollen  grains  are  in  reality  the  one-celled  stages  of  the  male 
gametophytes,  since  they  have  the  reduced  or  gametophytic 
number  of  chromosomes.  Usually  before  the  pollen  grain 
leaves  the  anther  its  nucleus  divides,  forming  a  tube  and  genera- 
tive nucleus.     In  this  condition  the  pollen  grain  is  carried  to  the 


462 


SPERMATOPHYTES   (SEED  PLANTS) 


stigma  where  the  male  gametophytc  completes  its  development. 
The  history  of  the  pollen  is  shown  in  the  upper  diagram  of 
Figure  409. 

The  Pistil.  —  A  i)istil  consists  of  one  or  more  megasporophylls 
(carpels).     The   megasporophyll   is   usually   organized   into   an 


Fig.  409.  —  The  formation  of  the  spores  and  gametophytes  m  Angiosperms. 
The  upper  diagram  shows  the  origin  of  the  pollen  grains  and  male  gameto- 
phjrtes.  a,  cross  section  of  a  young  anther,  showing  the  mother  cells;  b,  a 
mother  cell  beginning  to  divide;  c,  the  first  division  of  the  mother  cell  com- 
pleted; (/,  the  second  division  of  the  mother  cell  completed,  resulting  in  a  tetrad 
of  daughter  cells;  c,  cells  of  the  tetrad  separated  and  fully  formed  pollen  grains; 
/,  pollen  grain  with  male  gametophyte  developed,  showing  tube  nucleus  at  t, 
and  sperms  at  s. 

The  lower  diagram  shows  the  formation  of  megaspores  and  female  game- 
tophyte. g.  section  through  an  ovule,  showing  the  megaspore  mother  cell 
with  chromatin  in  a  thread  in  preparation  for  the  reduction  division;  h,  a  sec- 
tion through  an  ovule,  showing  the  four  megaspores  resulting  from  the  two 
successive  divisions  of  the  megaspore  mother  cell;  i,  section  through  an 
ovule  showing  the  mature  female  gametophyte  which  is  formed  by  the  sur- 
viving megaspore. 


ovary,  style,  and  stigma.  In  compound  pistils,  where  a  number 
of  carpels  are  present,  the  ovaries  are  usually  joined,  thus  form- 
ing a  compound  ovary,  and  often  the  styles  and  sometimes  the 
stigmas  are  also  joined. 

The  ovary,  which  is  the  enclosure  for  the  megasporangia  or 
ovules,  is  one  of  the  notable  features  of  Angiosperms.  With  the 
ovules  enclosed  the  pollen  cannot  come  in  contact  with  the  ovules 
as  it  does  in  Gymnosperms,  so  the  stigma,  another  characteristic 


^  THE  PISTIL  463 

structure  of  the  Anj>;iosperms,  had  to  be  introcUicecl.  That  the 
ovary  gives  the  Angiosperms  a  special  economic  importance  is 
attested  by  the  fact  that  our  fruits  are  either  ripened  ovaries  or 
ripened  ovaries  plus  closely  related  parts.  Within  the  ovary 
occur  the  cavities  or  locules  in  which  are  borne  the  megasporangia 
or  ovules,  varying  in  number  and  also  in  the  way  they  are  at- 
tached in  different  Angiosperms. 

The  ovule  is  generally  borne  on  a  stalk  (funiculus),  and  the 
chief  structure  of  the  ovule  is  the  nucehus,  which  in  most  Angio- 
sperms is  enclosed  by  two  integuments,  an  inner  and  an  outer  one. 
As  in  Gymnosperms,  the  integuments  do  not  completely  close 
over  the  top  of  the  nucellus,  but  leave  a  small  opening  (micropyle). 

Usually  the  ovule  curves  as  it  develops  and  the  micropyle  is 
brought  around  to  near  the  base  of  the  ovule.  This  position  of 
the  micropyle  is  a  favorable  one  for  the  entrance  of  the  pollen 
tube.  There  are  terms  used  to  indicate  the  amount  of  curving 
ovules  undergo  in  their  development.  Ovules  that  remain 
straight  are  orthotropous.  Those  that  double  clear  back  upon 
themselves  are  anatropous.  Those  turning  only  part  way  back 
upon  themselves  are  campijlotropous.  Within  the  nucellus  is 
formed  the  megaspore  mother  cell  {B,  Fig.  4O8),  which  also 
divides  by  two  successive  divisions  in  one  of  which  the  number  of 
chromosomes  is  reduced  to  the  gametophytic  number.  A  mega- 
spore, therefore,  produces  four  megaspores,  each  of  which  is  com- 
parable to  a  pollen  grain.  Although  the  megaspores  are  formed 
while  the  flowers  are  mere  buds,  they  are  formed  later  than  the 
pollen  grains.  As  in  the  Gymnosperms,  in  most  Angiosperms 
only  one  of  the  megaspores  develops  into  a  gametophyte,  although 
among  Monocotyledons,  there  are  cases  in  which  more  than  one 
or  all  of  the  megaspores  apparently  take  part  in  forming  the  one 
gametophyte.  The  lower  diagram  in  Figure  409  gives  the  usual 
history  of  the  megaspores. 

The  female  gametophtjte  is  very  much  reduced,  consisting  of 
only  a  few  nuclei  and  naked  cells  in  a  small  mass  of  cytoplasm. 
In  most  Angiosperms  the  female  gametophyte  is  developed  in 
the  following  way.  The  megaspore  first  enlarges  by  digesting 
and  using  the  other  three  megaspores  and  the  adjoining  cells  of 
the  nucellus  as  food.  Then  as  the  megaspore  further  enlarges 
the  nucleus  divides,  and  the  daughter  nuclei  pass  to  opposite 
ends  of  the  embryo  sac  which  is  the  term  now  applied  to  the 


464 


SPERMATOPHYTES   (SEED   PLANTS) 


region  enclosed  within  tlic  cell  membrane  of  the  germinating 
megaspore.  In  this  position  nuclear  division  follows  until  there 
are  four  nuclei  at  each  end.  The  megaspore  has  now  become  the 
female  gamotophyte  consisting  of  eight  nuclei,  four  at  the 
micropylar  and  four  at  the  opposite  end,  known  as  the  chalazal 
or  antipodal  end  of  the  embrj^o  sac.  After  the  stage  with  eight 
nuclei  is  reached,  then  the  organization  of  the  female  gameto- 
phyte  begins  as  shown  in  Figure  4^0.     A  nucleus  called  polar 

nucleus  from  each  end  of  the 
embryo  sac  moves  toward 
the  center  of  the  sac  until  the 
two  come  in  contact.  Some- 
times they  fuse  soon  after 
coming  in  contact  to  form 
the  primaiy  endosperm  nu- 
cleus, but  often  they  remain 
in  contact  until  fertilization 
and  then  fuse  at  the  same 
time  they  fuse  with  the 
sperm  to  form  the  endo- 
sperm nucleus.  The  three 
nuclei  and  adjacent  cyto- 
plasm at  the  micropylar  end 
are  organized  into  three 
naked  cells,  the  inner  one 
being  the  egg  and  the  other 
two  the  synergids.  The  three 
nuclei  at  the  antipodal  end 
and  known  as  antipodals 
usually  disappear  early,  but 
in  some  Angiosperms  they 
become  organized  with  the  adjacent  cytoplasm  into  cells  that 
seem  to  have  an  absorptive  function.  The  female  gametophj^te 
is  now  organized  and  ready  for  fertilization.  When  compared 
with  the  female  gametophyte  of  the  Pine,  its  remarkable  reduc- 
tion in  number  of  cells,  the  absence  of  archegonia,  and  the  forma- 
tion of  a  nucleus  for  providing  endosperm  are  notable  features. 
Male  Gametophyte  and  Fertilization.  —  On  the  stigma  the 
pollen  grain  ch^velops  a  tu])o  which  l)_y  means  of  enzymes  eats  its 
way  through  the  stigma,  style,  and  ovule  into  the  embryo  sac. 


Fig.  410.  —  Organization  of  the  female 
gametophj-te  in  Red  Clover.  At  the  left, 
a  section  through  the  nucellus,  showing 
eight  nuclei  of  the  female  gametophyte 
with  four  nuclei  at  each  end  of  the  em- 
bryo sac.  At  the  right,  the  gametophyte 
fully  organized,  showing  the  antipodals 
at  a,  the  polars  at  p,  the  egg  at  e,  and 
the  synergids  at  s. 


MALE   GAMETOPHVTE   AND   FEirilLlZATlON 


465 


The  pollen  tube  lives  as  a  parasite  on  the  structures  tlu-ough 
which  it  passes,  using  their  tissues  as  food  for  growth  and  mak- 
ing a  passageway  for  itself  at  the  same  time.  The  growth  of  the 
pollen  tube  is  directed  by  the  tube  nucleus  which  maintains  a 
position  near  the  end  of  the  tube.  Soon  after  the  pollen  tube  is 
well  started,  the  generative  nucleus  passes  from  the  pollen  grain 
into  the  tube  and  later  divides,  forming  two 
sperms  which  are  carried  along  with  the  con- 
tents of  the  tube  to  the  embryo  sac.  The 
male  gametophyte,  consisting  of  tube  nucleus 
and  two  sperms,  is  now  complete.  In  some 
plants,  however,  the  formation  of  the  sperms 
occurs  before  the  development  of  the  tube  is 
begun. 

When  the  tube  reaches  the  embryo  sac  and 
comes  in  contact  with  its  contents,  the  mem- 
brane enclosing  the  tube  is  destroyed,  and  the 
tube  nucleus,  sperms,  and  other  contents  of 
the  tube  flow  into  the  embryo  sac.  The  con- 
tents of  the  embryo  sac  apparently  destroy 
the  tube  nucleus,  for  it  soon  disappears,  while 
the  sperms  apparently  thrive.  Since  there 
are  no  cell  walls  in  the  embryo  sac,  the  sperms 
are  free  to  move  about.  As  to  how  they  are 
moved  is  not  known,  for  they  have  no  cilia, 
but  one  very  soon  reaches  the  nucleus  of  the 
egg  and  the  other  the  polar  nuclei  or  the 
primary  endosperm  nucleus,  with  which  they 
come  in  contact  and  fuse.  Since  there  are  two 
fusions,  one  with  the  egg  nucleus  and  the 
other  with  the  polar  nuclei  or  the  primary 
endosperm  nucleus,  there  are  two  fertilizations 
or  double  fertilization,  and  this  also  is  a  notable 
feature  of  Angiosperms  {Fig.  411).  Of  course  fertilization  is 
difficult  to  follow  and  has  been  seen  in  only  a  comparatively  few 
Angiosperms.  It  is  therefore  possible  that  many  times  the 
second  sperm  does  not  fuse  with  the  polars  or  the  primary 
endosperm  nucleus,  but  double  fertilization  has  been  found  so 
generally  in  the  Angiosperms  whose  fertilization  has  been  studied 
that  it  is  believed  to  be  cjuite  universal  among  Angiosperm.     In 


Fig.     411. —An 

embryo  sac  of  a 
Lily,  showing 
double  fertilization. 
At  the  upper  end  of 
the  sac  the  egg  (e) 
and  a  sperm  (s)  are 
shown  fusing,  and 
near  the  center  of 
the  .sac  the  second 
sperm  (s)  Is  sljown 
fusing  mth  the  two 
polar  nuclei  (p). 


466 


SPERMATOPHYTES   (SEED  PLANTS) 


connection  with  double  fertilization  it  should  be  noted  that  the 
endosperm  nucleus  contanis  the  contents  of  three  nuclei,  since 
it  is  a  product  of  a  triple  fusion,  involving  a  sperm  and  the  two 
polar  nuclei. 

Embryo.  —  The  first  cells  produced  by  the  division  of  the  fer- 
tilized egg  form  a  filament  which  pushes  down  into  the  embryo 
sac.  This  filament  is  called  the  proembryo.  The  terminal  cell 
of  the  proembryo  develops  the  embryo,  while  the  remainder  of 
the  filament  remains  as  a  stalk  called  suspensor.     After  the  termi- 


FiG.  412.  —  Development  of  the  embryo  and  endosperm  in  the  Shepherd's 
Purse.  A,  section  through  ovule  with  embryo  and  endosperm  in  early  stage 
of  development,  showing  the  proembryo  which  consists  of  the  suspensor  (6) 
and  the  terminal  three-celled  embryo  (a),  and  also  showing  the  endosperm  (c) 
as  a  chain  of  free  nuclei  around  the  wall  of  the  embryo  sac.  B,  the  same  as 
A,  excepting  that  the  proembryo  and  endosperm  are  more  developed.  C, 
section  through  a  mature  seed  showing  the  seed  coat  (s),  and  the  mature 
embryo  with  cotyledons  at  h,  plumule  at  p,  hypocotyl  at  e,  and  radicle  at  d. 


nal  cell  divides  a  number  of  times,  the  parts  of  the  embryo  begin 
to  be  differentiated.  In  Dicotyledons  two  lobes  appear  at  the 
end  farthest  from  the  micropyle  and  these  become  the  two  coty- 
ledons characteristic  of  dicotyledonous  Angiosperms.  Between 
the  cotyledons  the  plumule  is  formed,  while  the  axis  of  the  embryo 
below  the  cotyledons  is  differentiated  into  the  hypocotyl,  which 
is  the  main  part  of  the  axis,  and  the  radicle  at  its  lower  end 
(Fig.  412). 

The  embryos  of  monocotyledonous  Angiosperms  have  a  radicle, 
hypocotyl,  plumule,  ])ut  only  one  cotyledon.     They  also  differ 


POI.YEMBRYONY 


467 


from  the  embryos  of  Dicotyledons  in  the  relative  positions  of  the 
cotyledon    and    j^lumule.     Although    the    cotyledon    ai^parently 
arises  laterally,  it  soon  becomes  terminal  and  the  plumule  appears 
to  develop  on  the  side  of  the  em- 
bryo {Fig.  413). 

Parthenogenesis.  —  Partheno- 
genesis, which  is  the  develop- 
ment of  an  embryo  from  a  sup- 
posedly unfertilized  egg,  occurs 
in  a  number  of  Angiosperms. 
In  the  Dandelion  {Taraxacum), 
Meadow  Rue  {Thalidrum) ,  Ever- 
lasting {Antennaria) ,  Apples, 
Pears,  Quinces,  and  a  few  other 
plants  parthenogenesis  is  known 
to  occur.  In  cases  which  have 
been  investigated  cytologically,  it 
has  been  found  that  the  mother 
cell  in  the  ovule  omits  the  reduc- 
tion division,  and,  therefore,  the 
cell  which  occupies  the  position  of 
an  egg  has  the  sporophytic  num- 
ber of  chromosomes  and  fertiliza- 
tion is  not  necessary.  Since  par- 
thenogenetic  plants  show  no  re- 
sults of  crossing  in  the  offspring 
when  cross-pollinated,  partheno- 
genesis may  be  a  source  of  disap- 
pointment to  the  plant  breeder. 

Parthenocarpy.  —  Parthenocarpy  is  the  development  of  fruit 
without  fertilization  and  is  quite  common  among  Angiosperms. 
Bananas,  seedless  Oranges,  and  seedless  Currants  are  familiar 
examples  of  parthenocarpic  plants.  Sometimes  Apples  develop 
without  seeds,  and  some  varieties  of  Cucumbers  develop  fruits 
without  pollination. 

Polyembryony.  —  In  a  few  Angiosperms,  of  which  one  of  the 
Onions  {AUium)  is  a  notable  example,  a  number  of  embryos  ma}^ 
be  developed  in  the  same  embryo  sac  or  around  it.  The  syner- 
gids  and  antipodals  have  been  known  to  develop  embryos,  and 
sometimes  some  of  the  cells  of  the  nucellus  around  the  embryo 


Fig.  413.  —  A  inonocotyledon- 
ous  embryo  as  typified  by  that  of 
Corn.  The  cotyledon  (c)  appears 
terminal  and  the  plumule  (p)  as 
arising  from  the  side  of  the  em- 
bryo. 


468  SPERMATOPHYTES   (SEED   PLANTS) 

sac  develop  like  buds  and  form  embryos,  in  which  case,  of  course, 
there  is  no  fertilization.  Polyembryony  may  also  be  a  source  of 
annoyance  to  plant  breeders,  for  if  plants  that  are  used  in  cross- 
ing develop  polyembryonous  seeds,  the  offspring  arising  from 
these  seeds  may  develop  from  embryos  that  were  formed  by  the 
budding  of  the  nucellus,  in  which  case  the  embryos  have  only  the 
characteristics  of  the  mother  plant.  For  example,  in  crossing 
different  strains  of  Tobacco,  in  some  cases  the  plants  arising 
from  the  seeds  obtained  by  crossing  are  not  hybrids  but  like  the 
mother  plant.  Some  think  this  may  be  due  to  parthenogenesis 
and  others  attribute  it  to  polyembryony. 

Endosperm.  —  While  the  embryo  is  developing,  the  endosperm 
nucleus  is  dividing  and  its  accompanying  cytoplasm  is  increasing. 
The  free  nuclei  at  first  form  in  a  chain  around  the  wall  and  then 
multiply  towards  the  center.  Cell  walls  are  finally  formed  and  in 
these  cells  food  is  stored.  In  some  Angiosperms  the  endosperm 
is  taken  up  by  the  embryo  almost  as  rapidly  as  formed  and  stored 
in  the  cotyledons,  while  in  other  Angiosperms  most  of  the  en- 
dosperm remains  outside  of  the  embryo  until  the  seed  germi- 
nates. 

Since  the  endosperm  nucleus  contains  the  contents  of  a  sperm, 
the  character  of  the  endosperm  of  a  seed  is  often  determined  by 
the  sperm.  Thus,  as  in  case  of  Corn  where  the  endosperm  re- 
mains outside  of  the  embryo,  the  color  and  other  characteristics 
of  the  endosperm  are  often  like  the  pollen  parent  and  not  at 
all  like  those  of  the  mother  parent.  This  feature  called  xenia 
has  already  been  referred  to.  In  some  seeds,  in  addition  to  the 
formation  of  endosperm,  the  portion  of  the  nucellus  remaining 
becomes  stored  with  food  and  forms  what  is  known  as  peri- 
sperm. 

Seed  Coat.  —  As  the  embryo  and  endosperm  develop,  the 
ovule  enlarges  rapidly,  and  at  the  same  time  the  embryo  sac  de- 
stroys much  or  all  of  the  nucellus  and  frequently  a  part  or  all 
of  the  inner  integument.  Consequently  the  seed  coat  consists 
chiefly  of  the  outer  integument,  which  is  usually  very  much  mod- 
ified for  protection. 

It  is  obvious  that  the  seeds  of  Angiosperms  differ  considerably 
from  the  seeds  of  Gymnosperms,  for  the  female  gametophyte  of 
Angiosperms  is  soon  destroyed  after  fertilization  by  the  develop- 
ing embryo  and  endosperm,  and  consequently  there  is  no  gameto- 


SEED   COAT 


469 


phytic  tissue  in  the  seeds  of  Angiospcnns  coiiiparal)lc  to  that 
in  the  seeds  of  Gymnosperms.  The  endosperm  in  the  seeds  of 
Gymnosperms  is  simply  the  portion  of  the  gametophyte  that 


Fig.  414.  —  The  life  cycle  of  Aiigiosperms  illustrated  by  the  life  cycle  of 
Red  Clover.  At  the  left  in  the  line  above,  a  branch  of  Red  Clover  with 
heads  of  flowers  ( X  i) ;  next,  a  vertical  section  through  a  flower,  showing  the 
floral  structures;  at  the  right,  a  section  of  an  anther,  a  pollen  grain,  and  a 
pollen  grain  with  tube  and  male  gametophyte  developed.  At  the  left  in  the 
line  below,  an  ovule  with  female  gametophyte  mature  and  pollen  tubes  en- 
tering through  the  micropyle;  next,  embryo  and  endosperm  forming;  next, 
seed  mature  from  the  embryo  of  which  the  new  plant  at  the  right  develops. 


remains,  but  in  Angiosperms  the  endosperm  develops  after  the 
gametophyte  is  formed  and  from  a  nucleus  formed  by  the  fusion 
of  three  other  nuclei,  one  of  which  came  from  the  male  gameto- 
phyte. 


470  SPERMATOPHYTES   (SEED   PLANTS) 

Notable  Features  of  Angiosperms.  —  In  contrast  to  Gymno- 
spernis,  the  Angiosperms  have  the  flower;  a  megasporophyll  con- 
sisting of  an  ovary,  in  which  the  megasporangia  are  enclosed,  and 
of  a  stigma  to  receive  the  pollen;  more  reduced  gametophytes; 
and  endosperm  nucleus  and  double  fertilization.  The  hfe  cycle 
of  an  Angiosperm  is  shown  in  Figure  414. 


CHAPTER  XX 

CLASSIFICATION   OF   ANGIOSPERMS   AND    SOME 

OF   THEIR   FAMILIES   OF   MOST   ECONOMIC 

IMPORTANCE 

Classification.  —  The  Angiosperms  are  so  numerous  and  vary 
so  widely  that  their  classification  is  not  at  all  settled.  Ray,  a 
noted  English  botanist  (1628-1705),  divided  the  Angiosperms 
into  two  sub-classes  —  Monocotyledons  and  Dicotyledons  — 
on  the  basis  of  the  number  of  cotyledons.  There  are  also  other 
features  which  are  used  in  distinguishing  these  two  groups, 
such  as  the  number  of  floral  structures  composing  the  flower, 
the  venation  of  the  leaves,  the  arrangement  of  the  vascular 
bundles  in  the  stem,  and  the  presence  or  absence  of  cambium. 
Thus  leaves  with  parallel  veins,  the  parts  of  the  flower  in  threes 
or  sixes,  the  scattered  arrangement  of  vascular  bundles  m  the 
stem,  and  closed  vascular  bundles  are  characteristic  of  Mono- 
cotyledons, while  leaves  with  net-veins,  floral  parts  in  fours  or 
fives,  vascular  bundles  arranged  in  a  circle  so  as  to  enclose  the 
pith,  and  indefinite  growth  by  means  of  a  cambium  are  charac- 
teristic of  Dicotyledons.  As  to  whether  the  Monocotyledons 
arose  from  the  Dicotyledons,  or  the  Dictyledons  from  the 
Monocotyledons  is  a  question  that  botanists  are  not  able  to 
answer  satisfactorily.  However,  recent  studies  of  the  young 
embryos  of  some  of  the  Monocotyledons  show  that  there  are 
two  cotyledons  present,  one  of  which  is  very  rudimentary. 
This  discovery  with  other  structural  and  historical  features  has 
given  rise  to  the  view  that  the  monocotyledonous  condition 
arose  from  the  dicotyledonous  condition  through  the  suppres- 
sion of  one  of  the  cotyledons.  This  means  that  the  gro\vth 
which  first  becomes  evident  at  the  top  of  the  developing  embryo 
as  two  points,  each  of  which  develops  into  a  cotyledon  in  Dicoty- 
ledons, became  concentrated  into  the  development  of  only  one 
point  which  consequently  develops  the  single  large  cotyledon 
characteristic  of  Monocotyledons. 

471 


472  ANGIOSPERMS 

Upon  differences  which  pertain  chiefly  to  the  flowers,  the 
Monocotyledons  and  Dicotyledons  are  subdivided  into  many 
groups. 

The  Monocotyledons  are  subdivided  into  8  or  10  orders  which 
are  in  turn  subdivided  into  about  42  families.  The  families  are 
subdivided  into  many  genera  and  the  genera  into  species  of 
which  there  are  about  25,000. 

The  Dicotyledons,  of  which  there  are  more  than  100,000  spe- 
cies, include  most  of  the  Angiosperms,  being  more  than  four 
times  as  numerous  as  the  Monocotyledons.  The  Dicotyledons 
are  divided  into  two  large  subdivisions  —  the  Archichlamydeae 
and  the  Sympetalae. 

The  Archichlamydeae  have  a  corolla  of  separate  petals  or  no 
corolla  at  all.  They  include  about  180  families  and  61,000 
species  of  Dicotyledons.  They  are  grouped  into  two  classes, 
one  of  which  has  apetalous  flowers,  that  is,  flowers  without 
petals,  and  the  other  of  which  has  polypetalous  flowers,  that  is, 
flowers  with  petals  present  and  free  from  each  other. 

The  Sympetalae  include  those  Dicotyledons  in  which  the 
petals  are  more  or  less  united.  There  are  about  50  families 
and  42,000  species  of  the  Sympetalae. 

In  arranging  the  orders  and  families  taxonomists  have  en- 
deavored to  follow  an  evolutionary  sequence.  The  rank  of  an 
order  or  family  depends  chiefly  upon  the  organization  of  its 
flowers.  Flowers  most  like  a  typical  strobilus,  that  is,  resem- 
bling most  the  strobili  of  Gymnosperms,  are  regarded  as  the 
simplest  of  flowers.  Thus  a  flower  without  any  perianth  is 
simpler  than  one  with  a  perianth.  Also  a  flower  with  parts 
arranged  spirally,  thus  having  parts  arranged  like  the  sporophylls 
in  a  strobilus,  is  considered  simpler  than  one  with  parts  having  a 
cyclic  arrangement.  Again  flowers  having  petals  joined  or  carpels 
united  are  considered  more  advanced  than  flowers  in  which  these 
parts  are  separate.  Thus  the  Sympetalae,  since  they  have  united 
petals,  are  considered  more  advanced  than  the  Archichlamydeae 
which  have  separate  petals  or  no  petals  at  all.  In  respect  to 
these  evolutionary  tendencies  the  orders  and  families  of  both 
Dicotyledons  and  Monocotyledons  form  an  ascending  series. 

Most  of  the  families  of  the  Angiosperms  have  some  species 
of  economic  importance,  but  some  families  are  much  more 
notable  than  others  for  their  species  related  to  man's  welfare. 


(WILLOW   FAMILY   SALICACEAE) 


473 


The  species  nuiy  concern  us  because  they  are  useful  for  food, 
fibers,  lumber,  medicine,  etc.,  or  because  they  are  weeds  which 
hinder  the  growth  of  cultivated  plants,  poison  live  stock,  or  do 
damage  in  other  ways. 

Beginning  with  one  of  the  lower  families  of  the  Dicotyledons, 
a  number  of  families  of  Angiosperms  having  species  of  consid- 
erable economic  importance  are  discussed  in  the  following  pages. 


Fig.  415.  —  The  flowers  ol  a  Willow.  Above,  at  the  left,  a  staminate 
catkin,  and  below,  at  the  left,  a  staminate  flower,  showing  the  bract  and  sta- 
mens; above,  at  the  right,  a  pistillate  catkin,  and  below,  at  the  right,  a  pistil- 
late flower,  showing  the  bract  and  pistil.     After  Burns  and  Otis. 


Archichlamydeae 

A  petalae 

Willow  Family  (Salicaceae) .  —  This  family,  although  it  is  not 
the  lowest  family  of  the  Dicotyledons,  stands  well  toward 
the  bottom  of  the  series.  To  this  family  belong  the  Willows 
and  Poplars.  The  flowers  are  unisexual  and  simple  in  type. 
The  plants  are  dioecious  and  bear  their  apetalous  flowers  in 
scaly  spikes  or  catkins  {Fig.  4^5).     A  flower  consists  of  a  pistil 


474 


ANGIOSPERMS 


or  of  a  nuinl)(M'  of  staiii(>ns  boriu^  in  the  axil  of  a  small  scale  or 
bract. 

The  AVoepin^  Willow,  so  named  because  of  its  drooping 
branches,  is  cultivated  for  its  beauty.  The  growing  of  Basket 
Willows  for  sprouts,  which  are  woven  into  baskets,  chairs,  and 

other  articles,  is  an  industry 
of  considerable  importance. 
Willows  are  easily  propagated, 
taking  root  readily  when 
transplanted  or  from  cuttings. 
They  grow  especiall}^  well  near 
water  and  are  often  planted 
along  river  banks  where  they 
prevent  the  cutting  away  of 
the  banks  by  floods.  A  num- 
ber of  the  Poplars,  such  as 
the  Aspens,  Balm  of  Gilead, 
and  Cottonwood,  are  culti- 
vated for  shade.  The  Cot- 
tonwood grows  to  be  a  very 
large  tree  and  is  of  some 
value  for  lumber.  Both  Wil- 
lows and  Poplars  are  used  in 
making  medicinal  charcoal, 
and  a  number  of  substances, 
such  as  salicin,  populin, 
tannin,  and  a  volatile  oil  are 
obtained  from  their  bark. 

"Walnut  Family  (Juglan- 
daceae).  —  This  family  com- 
prises the  Walnuts  and  Hick- 
ories. The  Walnuts  and 
Hickories  are  monoecious,  and  their  flowers  are  generally  apetal- 
ous,  although  in  some  cases  the  pistillate  flowers  have  petals 
The  staminate  flowers  are  borne  in  catkins,  while  the  pistillate 
flowers  are  borne  singly  or  in  small  clusters  {Fig.  4^6)- 

The  White  Walnut  {Juglans  cinerea),  called  Butternut,  and  the 
Black  Walnut  {Juglans  nigra)  are  the  most  common  Walnuts  in 
the  United  States.  The  European  Walnut  {Juglans  regia),  not- 
able for  its  delicately  flavored  nuts,  is  grown  in  California  and 


Fig.  416.  —  The  flowers  and  fruit 
of  the  Black  Walnut.  At  the  left,  a 
branch  bearing  a  catkin  of  staminate 
flowers  below  and  two  pistillate  flowers 
above  (X|).  At  the  right,  above,  a 
pistillate  flower,  shownng  the  pistil 
enclosed  in  bracts  which  form  the  husk 
of  the  fruit;  next,  below,  a  staminate 
flower,  showing  the  bracts  and  the 
stamens;  at  the  bottom,  a  fruit  (X5). 
After  Burns  and  Otis. 


BIRCH   FAMILY 


475 


the  Southern  States,  and  some  other  species  occur  in  certain 
parts  of  the  United  States. 

The  nuts  are  rich  in  oil,  which  is  expressed  and  used  as  food 
and  in  painting.     The  nuts  are  common  on  the  market  and  are 
of  considerable  importance  as  food.     The  wood  of  the  White 
and  Black  Walnut  is  much 
used  for  furniture  and  cab- 
inet work.    The  wood  of  tlu; 
Black   Walnut    is    probably 
the  most  valuable  wood  of 
the  North  American  forest. 
It  is  a  durable  wood,  takes 
a  fine  polish,   and  is   much 
sought    for    furniture,    gun- 
stocks,     and     for     cal^inet 
work. 

There  are  a  number  of 
species  of  Hickories,  and 
the  Pecans  and  several  other 
species  bear  nuts  having 
considerable  value  for  food. 
Hickory  wood  is  very  tough, 
and  on  account  of  its 
strength,  elasticity,  and 
lightness,  it  is  the  best  wood 
for  spokes  of  buggy  and 
wagon  wheels  and  for  ax 
handles.  It  is  also  the  best 
wood  for  fuel. 

Birch  Family  (Betulaceae), 
—  To  this  family  belong  tlie 
Birches,  Hazelnuts,  Iron- 
woods,  and  Alders.  They  are  trees  or  shrubs  and,  except  in 
rare  cases,  are  monoecious  with  the  staminate  flowers  borne  in 
catkins,  and  the  pistillate  flowers  borne  in  clusters,  in  spikes, 
or  scaly  catkins  {Fig.  417).  The  fruit  is  a  one-seeded  nut, 
which  in  the  Hazel  is  of  some  value  for  food.  The  Birches,  of 
which  there  are  many  species,  are  the  most  important  genera 
in  this  family.  They  are  much  used  for  shade  and  ornamental 
trees,  and  the  wood  is  used  for  furniture,  barrel  hoops,  shoe  pegs, 


Fig.  417. — ^  The  flowers  and  fruit  of 
the  Cherry  Birch.  At  the  left,  above, 
a  flowering  branchlet  bearing  two  stam- 
inate catkins  at  the  left  and  one  pis- 
tillate catkin  at  the  right  (X5);  at  the 
right,  above,  a  pistillate  flower  and  just 
below  a  staminate  flower;  at  the  left, 
below,  a  pistillate  catkin  in  fruit  and  at 
the  right,  below,  a  single  fruit.  After 
Burns  and  Otis. 


476 


ANGIOSPERMS 


Fig.  418.  —  The  flowers  and 
fruit  of  the  Red  Oak.  Above, 
a  flowering  branchlet  bearing  a 
cluster  of  staminate  catkins  be- 
low and  solitary  pistillate  flowers 
above  ( X  ^) ;  at  the  right,  above, 
a  pistillate  flower,  and  just  be- 
low, a  staminate  flower;  at  the 
bottom,  a  mature  fruit,  showing 
the  matured  ovary  and  the  cu- 
pula (natural  size).  After  Burns 
and  Otis. 

From  the  Cork  Oak  the  cork 


spools,  and  paper  pulp.  The  bark 
of  the  Paper  Birch  was  employed 
by  the  Indians  for  canoes^  baskets, 
cups,  and  for  sheathing  wigwams. 

Beech  and  Oak  Family  (Faga- 
ceae).  —  This  family  includes  the 
Beeches,  Chestnuts,  and  Oaks. 

The  plants  of  this  family  are 
monoecious  trees  or  shrubs  with 
staminate  flowers  in  catkins  or 
clusters,  and  pistillate  flowers  soli- 
tary or  slightly  clustered  {Fig.  418). 
The  fruit  is  a  one-seeded  nut 
partly  or  entirely  enclosed  by  a 
covering  called  cupule,  which  is 
formed  by  bracts  that  develop  at 
the  base  of  the  ovary  and  grow 
up  over  it. 

The  nuts  of  the  Chestnut  are 
common  on  the  market  and  are  of 
considerable  value  for  food.  Beech 
nuts  contain  much  oil  and  are  a 
good  feed  for  hogs.  From  the 
Oaks,  of  which  there  are  a  large 
number  of  species,  a  large  propor- 
tion of  our  hardwood  is  obtained. 
The  beautiful  figures  which  Oak 
lumber  can  be  made  to  show  make 
it  a  valuable  wood  for  furniture, 
inside  finishing  of  buildings,  and  for 
cabinet  work.  Beech  wood  is  very 
hard  and  is  used  considerably  for 
hardwood  floors  and  in  the  manu- 
facture of  furniture.  Chestnut  wood 
is  soft  but  durable  and  is  used  for 
fences  and  buildings.  The  bark  of 
Oak  and  Chestnut  trees  is  rich  in 
tannin  and  at  one  time  was  the 
source  of  tannin  for  tanning  hides, 
of  commerce  is  obtained  (Fig.  419). 


BEECH   AND  OAK   FAMILY    (FAGACEAE) 


477 


Fig.  419.  —  Stripping  cork  from  the  Cork  Oak.     After  Lecomte. 


Fig.  420.  —  The  flowers  and  fruit  of  the  Red  Mvdberry.  Above,  from  left 
to  right,  a  spike  of  staminate  flowers,  a  spike  of  pistdlate  flowers,  and  a  pis- 
tillate spike  in  fruit  (natural  size) ;  at  the  bottom,  a  staminate  and  pistillate 
flower  much  enlarged.     After  Burns  and  Otis. 


478 


ANGIOSPERMS 


Elm  Family  (Urticaceae).  —  The  Elm  family  includes  about 
1"){)()  spccit'S  of  herbs,  shrubs,  and  trees.  Besides  the  Elms  this 
family  includes  the  Mulberries,  Figs,  Hemps,  Hops,  Nettles, 
tropical  Bread  Fruits  and  a  number  of  others  less  important. 

The  apetalous  flowers  are  mostly  unisexual.  The  flowers 
are  usually  borne  in  loose  or  catkin-like  clusters  (Fig.  Jf20).  In 
the  Fig  the  flowers  are  produced  in  hol- 
low receptacles,  which  with  the  ovaries 
within  form  the  well-known  fleshy  fruits 
of  the  Fig  {Fig.  421).  The  fruits  in  this 
family  vary  much  in  size,  form,  and 
texture.  In  the  Elms  the  fruits  are 
winged  and  depend  upon  the  wind  for 
dissemination. 

The  Elms  are  very  popular  shade  trees, 
and  their  wood  is  used  for  flooring,  hubs, 
barrels,  sills,  posts,  and  railroad  ties. 
The  multiple  fleshy  fruits  of  the  Mul- 
berries are  edible,  and  the  leaves  of  Mul- 
berries constitute  the  food  for  silkworms. 
The  Hemps  are  well-known  fiber  plants, 
and  the  Hop  Vine  is  extensively  grown 
for  its  fruits,  which  are  used  in  br3wing  beer  and  at  one  time 
were  used  in  making  bread.  The  Rubber  Plant,  so  common 
in  greenhouses  and  homes,  belongs  to  this  family  and  is  one 
of  a  number  of  plants  that  yield  the  invaluable  rubber  from 
their  milky  juice. 

Buckwheat  Family  (Polygonaceae).  ^  The  plants  of  this 
family  are  mostly  herbs,  distinguished  by  their  swollen  nodes, 
sheathing  stipules,  and  simple  flowers  in  clusters  {Fig.  1^22). 
The  Smartweeds  and  Knotweeds,  which  are  extremely  common 
around  gardens  and  in  waste  places,  are  well-known  plants  of 
this  family.  The  fruit,  in  most  cases,  is  an  achene  which  is 
usually  angled  and  sometimes  winged.  In  case  of  Buckwheat^ 
which  is  an  important  cereal  crop,  the  starchy  achene  is  ground 
into  flour.  Some  of  them,  as  the  Rhubarb  and  Sorrel,  contain 
acid  in  the  leaves  or  stem.  The  family  includes  a  number  of 
we'eds  of  which  the  Docks,  Field  or  Sheep  Sorrel  {Fig.  423), 
Black  Bindweed,  Climbing  False  Buckwheat,  and  the  Smart- 
weeds  are  common  ones. 


Fig.  421.  — Pistillate 
flowers  of  the  Fig,  show- 
ing the  flowers  borne  in 
a  hollow  receptacle. 


BUCKWHEAT  FAMILY   (POLYGON ACE AE)  479 


Fig.  422.  —  A  Smartweed  {Polygonum  Muhlenhergii),  one  of  the  trouble- 
some weeds,  showing  the  sheathed  nodes  and  terminal  spikes  of  flowers  ( X  |), 
and  also  showing  a  flower  and  a  fruit  much  enlarged.  This  plant  has  both 
underground  and  aerial  stems. 


Fig.  423. — Field  or  Sheep  Sorrel  {Rumex  Acetosella),  showing  the  underground 
and  aerial  stems,  the  halberd-shaped  leaves,  and  terminal  spikes  of  flowers.   Xh 


480 


ANGIOSPERMS 


Goosefoot  Family  (Chenopodiaceae).  —  This  family  oontains 
many  plants,  chiefly  herbs  and  most  of  which  are  weeds.     The 


Fig.  424.  —  Russian  Thistle  {Salsola  Kali,  var.  tcnuiJuUa).  At  the  left, 
an  entire  jilant,  showing  the  tap-root  and  character  of  the  stem  (XiV);  at 
the  right,  a  portion  of  a  plant,  showing  the  leaves  and  flowers  about  natural 
size.     Modified  from  Oswald  and  from  Beal. 

flowers  are  small  and  usually  greenish.     The  Spinach  and  Beets 

are  well-known  pot  herbs  of  this 
family,  and  also  from  Beets  most 
of  our  sugar  is  now  obtained. 
Among  the  many  that  are  classed 
as  weeds,  the  Russian  Thistle 
{Fig.  Jf.2Ji.)  is  the  most  noted  one. 
Belonging  to  thesameorderisthe 
Amaranth  family,  which  contains 
some  ornamental  plants  and  a 
number  of  common  weeds.  Of  those 
that  are  ornamental,  the  Cocks- 
comb, Prince's  Feather,  and  Bache- 
lor's Button,  grown  in  gardens  for 
their  highly  colored  flower  clusters, 
are  common  ones.  The  Pigweed, 
and  Tumbleweed  (Fig.  425),  com- 
mon in  gardens,  truck  patches, 
and  waste  places,  are  the  most 
troublesome  weeds  of  this  family. 
Pink  Family  (Caryophyllaceae).  —  This  family  contains  many 

species,  which  are  chiefly  herbs  of  the  temperate  regions.     The 


Fig.  425.  —  The  Tumble  Weed 
(Amaranthus  graecizans),  showing 
the  general  character  of  the  plant. 

xi 


CROWFOOT   OR   BUTTERCUP   FAMILY 


481 


plants  are  like  those  of  the  preceding  families  in  character  of 
the  ovary  and  seeds  but  differ  from  them  in  having  a  perianth 
differentiated  into  a  showy  corolla 
and  a  large  calyx  (Fig.  1^.26).  They 
are  regarded  as  a  transition  group 
between  the  Apetalae  and  Poly- 
petalae.  Among  them  are  some 
garden  favorites,  such  as  the  Carna- 
tions, Pinks,  Sweet  Wilhams,  and 
Lychnis,  and  also  some  weeds  of 
which  the  Chickweeds,  Corn  Cockle, 
Cow-herb,  and  Bouncing  Bet  are 
common  ones. 


Polijpetalae 

As  previously  stated  the  Poly- 
petalae  have  petals  and  the  petals 
are  generally  separate.  The  colored 
corolla  is  usually  distinct  from  the 
green  calyx,  and  the  flowers  are  pol- 
linated chiefly  by  insects.  Among 
the  lower  families  of  the  Polypeta- 
lae,  as  the  Buttercups  (Ranunculaceae)  illustrate,  the  flower  usu- 
ally has  numerous  stamens  and  a  number 
of  separate  pistils.  The  calyx  and  corolla 
are  also  attached  below  the  stamens  and 
pistils  or,  in  other  words,  the  flowers  are 
hypogynous.  In  passing  to  the  more  ad- 
vanced families  of  the  Polypetalae,  the 
number  of  stamens  and  carpels  become 
more  definite,  and  assume  the  cyclic  ar- 
rangement. There  is  also  a  tendency  for 
the  carpels  to  join  and  a  tendency  of  the 
flower  toward  epigyny  in  the  higher 
families. 

Crowfoot  or  Buttercup  Family  (Ramrn- 
culaceae) .  —  This  family  includes  numer- 
ous species,  mostly  herbs,  having  in  common  separate  petals, 
and  separate  sepals.     The  stamens  and  connnonly  the  carpels 


Fig.  426.  —  A  portion  of  a 
plant  of  Corn""  Cockle  {Agro- 
stemma  Githago)  (X|).  The 
flowers  have  a  perianth  consist- 
ing of  a  calyx  and  showy  corolla. 
Modified  from  Beal. 


Fig.  427.  —  A  flower 
of  a  Buttercup,  .showing 
the  many  stamens  and 
pistils.      X2. 


482 


ANGIOSPERMS 


are  numerous,  but  indefinite  in  number  and  separate  (Fig.  427). 
A  few  of  the  well-known  plants  of  this  family  are  the  Anemone, 
Clematis,  Larkspur,  Columbine,  Hepatica,  Marsh  Marigold,  and 
Peony.  The  Wolfsbane  or  Aconite,  which  contains  the  virulent 
poison  aconite,  and  the  Golden  Seal,  which  yields  the  drug  hydras- 
tis,  are  medicinal  plants  of  considerable  importance.     Belonging 


Fig.  428.  —  American-grown    Camphor    trees.     From    Yearbook,    U. 
Dept.  .\gr. 


to  other  families  grouped  in  the  same  order  with  the  Buttercups, 
are  the  Magnolias,  trees  and  shrubs  noted  for  their  large  flowers 
and  including  the  Tulip  tree,  a  noted  timber  tree.  Also  the 
Barberries,  the  tropical  Nutmeg  tree,  and  the  Laurels  belong 
to  the  same  order.  The  Laurels  include  such  plants  as  the  Sas- 
safras, Cinnamon  and  Camphor  tree  (Fig.  428) 

Mustard   Family    (Cruciferae).— The   flowers   of    this   family 
generally  have  four  sepals,  four  petals,  and  six  stamens.     The 


ROSE  FAMILY 


483 


pistil  forms  a  pod  known  as  the  silique.  The  four  petals,  when 
opened  out,  suggest  the  (Jreek  cross,  —  whence  the  name  Cru- 
eiferae  {Fig.  429). 

To  this  family  belong  such  useful  plants  as  the  Cabbage, 
Turnip,  Kohlrabi,  Brussels  Sprouts,  and  Rape. 

A  number  of  plants  of  this  family,  such  as  Peppergrass, 
Shepherd's  Purse,  White  and  Black  Mustard,  Tumbling  Mus- 
tard,  Indian  Mustard,  and  Charlock  are  weeds.     Their  seeds 


Fig.  429. —  The  character  of  the 
plant,  flowers,  and  fruit  of  the  Black 
Mustard  {Brassica  nigra).  At  the 
right,  a  plant  in  flower  (X^^),  and  a 
mature  pod  about  natural  size;  at  the 
left,  above,  a  flower,  and  below,  an 
open  pod.     After  Vasey  and  Nature. 


Fig.  430.  — 
One  of  the  Pop- 
pies, showing 
the  character  of 
the  flowers  and 
pod.  After  Le- 
comte. 


are  troublesome  impurities  in  commercial  seeds,  and  the  seeds 
of  some  are  poisonous. 

Associated  with  the  Mustard  family  is  the  Poppy  family 
(Papaveraceae),  characterized  by  a  milky  juice  and  represented 
by  Blood  root,  common  in  the  woods,  and  by  the  California 
Poppy  from  the  juice  of  which  opium  is  obtained  {Fig.  430). 

Rose  Family  {Rosaceae).  —  To  this  family  belong  about  2000 
species  of  herbs,  shrubs,  and  trees.  In  most  plants  of  this 
family,  there  is  an  indefinite  number  of  stamens  and  one  to 
many  separate  carpels.  The  flowers  of  Strawberries  and  Black- 
berries, for  example,   have  many  stamens  and   many  separate 


484 


ANGIOSPERMS 


pistils,  and  the  number  of  each  is  indefinite,  while  in  the  Apple 
there  are  generally  five  united  carpels,  and  in  the  Peach,  Plum, 
Cherry,  and  Almond  the  number  of  carpels  has  settled  down  to 
one.  There  is  also  a  noticeable  tendency  toward  epigyny,  for 
perigyny,  which  is  common  in  the  family,  is  a  step  toward 
epigyny  (Fig.  431).  The  Rose  family  is  the  family  of  fruits. 
It  includes  Apples,  Pears,  Peaches,  Plums,  Apricots,  Cherries, 


Fig.  431.  —  Some  flowers  of  the  Rose  family.  At  the  left,  a  Strawberry 
flower,  which  has  many  stamens  and  pistils  and  is  hypogynous;  next,  a  flower 
of  an  Agrimony,  and,  at  the  extreme  right,  a  Pear  flower,  both  of  which  are 
perigynous  and  have  few  pistils  with  ovaries  joined. 


Quinces,  Strawberries,  Blackberries,  Raspberries,  and  some 
others.  No  one  can  estimate  what  this  family  contributes  to 
the  welfare  of  mankind. 

Some,  like  the  Roses,  Spireas,  and  Hawthornes,  are  impor- 
tant ornamental  plants.  Some  of  them,  as  the  Cinquefoils 
or  Five-fingers  and  the  Agrimonies,  are  weeds.  The  Five- 
fingers  grow  in  fields  and  crowd  out  other  plants,  while  the 
Agrimonies  grow  in  pastures,  and  their  spiny  fruits  get  in  the 
wool  and  hair  of  five  stock. 

Closely  related  to  the  Rose  family  is  the  Saxifrage  family 
(Saxifragaceae) ,  the  family  to  which  the  Gooseberry,  Currant, 
Syringa,  and  Hydrangea  belong. 

Pea  Family  {Leguminosae) .  — -The  Pea  family,  which  includes 
about  7000  species  of  herbs,  shrubs,  and  trees,  is  the  largest 
group  of  the  Archichlamydeae.  The  flowers  are  hypogynous 
or  somewhat  perigynous,  and  the  parts  of  the  calyx  and  corolla 
are  generally  in  fives  The  stamens  are  usually  10,  and  9  or  all 
of  them  are  joined.  The  petals  are  often  irregular,  as  those  of 
the  Beans  and   Peas  illustrate,  and   also  show  a  tendency  to 


PEA   FAMILY 


485 


unite  (Fig.  432).  In  the  uniting  of  some  of  the  petals,  the  plants 
of  the  Pea  family  suggest  those  of  the  Sympetalae  which  is 
considered  the  most  advanced  group  of  Dicotyledons.  Also 
irregularity  in  the  shape  or  size  of  sepals  or  petals  is  considered 
an  advanced  feature.  The  pistil  consists  of  one  carpel  and  be- 
comes the  one-celled  fruit  called  legume,  which  is  characteristic 
of  the  family. 

The  Beans,  Peas,  Peanuts,  Soy  Beans,  Cow-peas,  Clovers, 
Alfalfas,  and  Vetches  make  this 
family  a  noted  one.  The  value 
of  Beans,  Peanuts,  and  Peas  as 
food  for  man,  and  of  the  others 
mentioned  for  forage  and  the 
improvement  of  the  soil  are  well 
known  to  the  student.  The  Pea- 
nut is  peculiar  in  that  it  forces 
its  pods  underground  to  ripen. 
Although  Peanuts  are  not  so 
important  for  food  as  Beans  and 
Peas,  several  miUions  of  bushels 
of  them  are  grown  in  the  United 
States  per  year.  From  some 
of  the  leguminous  plants  medici- 
nal substances,  dyestuffs,  gum 
arable,  licorice,  logwood,  copal 
varnish,  and  other  useful  sub- 
stances are  obtained.  Some  of 
the  leguminous  trees,  as  the 
Black  Locust,  Honey  Locust, 
and  a  number  of  trees  in  the 
tropics  and  sub-tropics,  furnish 
fine  cabinet  woods. 

Thirty  or  more  leguminous  plants  are  classed  as  Aveeds. 
Some,  like  the  Loco-weeds  and  some  of  the  Lupines,  are  poison- 
ous to  live  stock  and  cause  considerable  trouble  in  pastures  in 
the  Western  states.  Some,  like  the  Rabbit-foot  Clover,  are  very 
hairy  and  when  eaten  by  stock,  the  hairs  often  collect  in  balls 
and  clog  the  intestines.  In  the  Tick  Trefoils,  of  which  there  are 
many  species,  the  fruits  are  commonly  spiny  and  are  trouble- 
some to  wool-growers. 


Fig.  432.  —  Flowers  and  fruit  of 
the  Common  Locust  {Robinia 
Pseudo- Acacia).  At  the  right,  a 
raceme  of  flowers,  showing  the 
irregular  corollas  ( X  |) ;  at  the  left, 
above,  a  flower  with  a  portion  of 
corolla  removed  to  show  the  diadel- 
phous  stamens;  at  the  left,  below, 
a  mature  pod  or  legiime  (X^). 
After  Burns  and  Otis. 


486 


ANGIOSPERMS 


Spurge  Family  {Euphorbiaceae). — The  Spurge  family  con- 
tains many  species,  many  of  which  are  tropical.  The  flowers 
are  commonly  small,  hypogynous,  and  unisexual.  The  perianth 
is  usually  simple  and  sometimes  absent.  The  stamens  range 
from  one  to  many,  and  the  pistil  is  composed  of  three  united 
carpels  (Fig.  433).  The  plants  usually  contain  a  milky  juice, 
which  in  many  species  is  poisonous.  A  few  of  them  are  common 
weeds,  usually  growing  prostrate  in  gardens  and  truck  patches. 


Fig.  433.  —  Flowers  and  fruit  of  the  Flowering 
Spurge  {Euphorbia  corollata).  At  the  right,  a  por- 
tion of  a  plant  in  flower;  above,  at  the  left,  a 
flower  cluster  consisting  of  one  pistillate  flower  and 
a  number  of  staminate  flowers  enclosed  by  an  in- 
volucre (i)  bearing  appendages  resembling  petals; 
at  the  right  of  the  flower  cluster,  a  single  stami- 
nate flower  with  anther  at  o;  below,  at  the  left, 
a  flower  cluster  with  staminate  flowers  removed 
to  show  the  pistillate  flower;  below,  at  the  right,  a 
pistillate  flower  in  fruit,  showing  the  ovary  (c),  the 
stigma  (s),  and  the  involucre  {i).  In  part  after 
Bergen  and  Caldwell. 


FiG.434.  — TheHevea 
tree,  one  of  the  plants 
from  the  milk-juice  of 
which  India  rubber  is  ob- 
tained.    After  Lecomte. 


The  Castor  Bean,  from  which  castor  oil  is  obtained,  is  one  of  the 
large  species  of  our  region.  Some  are  trees,  as  for  example  the 
Hevea  tree  {Fig.  43^)  of  South  America  from  which  India  rubber 
is  obtained.  Tapioca  is  obtained  from  the  Cassava  plant,  a 
plant  of  the  Spurge  family  and  native  of  Brazil.  A  number  are 
useful  for  medicine,  and  some,  as  the  Castor  Bean,  Poinsetta, 
and  some  others,  are  ornamental  plants. 

Between  the  Pea  family  and  the  Spurge  family  is  usually 
placed  the  Flax  family  (Lmaceae)  to  which  the  cultivated  Flax 


MALLOW  FAMILY 


487 


belongs,  and  the  Rue  family  (Rutacea),  the  family  of  citrous 
fruits,  such  as  Oranges,  Lemons,  Tangerines,  Grapefruit,  and 
others. 

Maple  Family  (Aceraceae).  —  This  family  is  composed  chiefly 
of  the  Alaples,  valuable  trees  for  shade,  lumber,  and  yielding  a 
sweet  sap  from  which  maple  syrup  and  sugar  are  obtained. 
Closely  related  to  the  Maples  are  the  Buckeyes  which  are  also 
important  shade  trees. 

Mallow  Family  (Malvaceae) .  —  This  family  is  a  notable  one 
chiefly  because  it  includes  the  Cotton  plant  (Fig.  435).     The 


yj 


■fA 


WW 


Fig.  435. — A  Cotton  Plant,  showing  the  gcicrai 
X  about  iV-     After  Orton. 


iracter  of  the  plant. 


flowers  have  five  sepals  and  five  petals.  The  stamens  are 
numerous  and  united,  and  the  pistil  is  composed  of  a  number 
of  carpels  united  at  the  base.  The  sepals  are  also  partlv  united 
(Fig.  9). 

Cotton  surpasses  all  other  plants  of  the  family  in  value. 
To  this  family  also  belongs  the  Theobroma  Cacao,  a  small  tree 
which  yields  cocoa  and  chocolate.  The  Shrubby  Althaea  and 
Hollyhock  are  of  some  importance  as  ornamental  plants,  while 
the  Indian  Mallow  (Alhutilon  Theophrasti) ,  Flower-of-an-hour 
(Hibiscus  Trionum),  and  a  few  others  are  more  or  less  trouble- 
some weeds. 


488 


ANGIOSPERMS 


Parsley  Family  (Umbelliferae) .  —  The  Parsley  family  com- 
prises about  1300  species.  The 
small  epigynous  flowers  are 
borne  in  umbels,  —  whence  the 
name  of  the  family  {Fig.  436). 
The  stamens  and  parts  of  the 
calyx  and  corolla  are  five.  The 
pistil  consists  of  two  partly 
united  carpels  which  separate 
in  the  fruit.  Carrots,  Parsnips, 
Celery,  and  Fennel  are  mem- 
bers of  this  family. 

This  family  also  contains  some 
bad  weeds.  The  poison  Hemlock 
{Conium  maculatum)  and  Water 
Hemlock  (Cituta  maculata)  are 
two  very  poisonous  plants, 
which  often  grow  in  pastures 
where  livestock  eat  them  and 
are  killed.  The  Wild  Carrot 
(Fig.  437)  is  troublesome  in  pas- 
tures, meadows,  and  grain  fields  where  it  crowds  out  other  plants. 


Fig.  436.  —  Flowers  and  fruit  of 
the  Wild  Carrot  {Daucus  Carota).  At 
the  left,  a  portion  of  a  plant  bearing 
umbels  of  flowers  and  fruit;  at  the 
right,  flowers  and  a  fruit  much  en- 
larged to  show  their  structure. 


Fig.  437.  —  a  meadow  taken  by  the  Wild  Carrot. 


HEATH   FAMILY 


489 


Sy77ij)etnlae 

Among  the  fifty  or  more  families  of  the  Sympetalae,  there  are 
some  famiUes  of  considerable  economic  importance.     As  previ- 
ously stated,  the  Sympetalae  are  characterized  by  a  gamopet- 
alous  corolla.    Also  the  ova- 
ries are  commonly  inferior. 
Their  flowers  are  commonly 
showy  and  insect  pollinated. 

Heath  Family  (Ericaceae). 
—  The  plants  of  the  Heath 
family  are  mostly  shrubs, 
and  they  are  distributed 
from  the  polar  regions  to  the 


Fig.  438.  —  One  of  the 
Bindweed .s  {Convolvulvus 
sepium),  showing  the  corolla 
composed  of  united  petals. 


Fig.  439. —  Alfalfa 
Dodder  twining  about  an 
Alfalfa  plant  and  drawing 
nourishment  from  it  by 
means  of  parasitic  roots 
(Xi).  Below,  at  the 
right,  also  a  fruit,  called 
capsule,  of  the  Dodder  is 
shown  much  enlarged. 


tropical  forests.  The  flowers  are  usually  regular,  and  both  calyx 
and  corolla  are  4-5  lobed.  The  stamens  are  as  many  or  twice 
as  many  as  the  lobes  of  the  calyx  or  corolla,  and  the  flowers  are 
liypogynous  or  perigynous. 

Some,  as  the  Cranberries,  Blueberries,  and  Huckleberries,  pro- 
duce berries  that  are  valuable  fruits.  The  Heath  family  also 
includes  some  highly  prized  ornamental  shrubs,  such  as  the  Rho- 


490 


ANGIOSPERMS 


Fig.  440.  —  A  portion  of  a  Tomato 
plant  bearing  flowers  and  fruits,  and  also 
a  flower  enlarged  to  show  the  structure 
of  the  flower. 


dodcndrons  and  Heathers. 
The  Traihng  Arbutus 
(Epigaea),  which  is  the 
favorite  spring  flower 
wherever  it  grows,  and  the 
Madrona,  one  of  the  most 
beautiful  trees  of  the 
Pacific  coast,  belong  to  this 
family. 

Sweet  Potato  Family 
(C  onvolvulacene).  —  The 
plants  of  this  family  are 
chiefly  trailing  or  twining 
herbs.  Their  flowers,  as 
those  of  the  Morning  Glory 
illustrate,  are  often  quite 
showy.  They  have  five 
stamens,  and  their  calyx 
and  corolla  are  composed 
of  five  parts  {Fig.  J^8). 
There  is  usually  one  pistil 
with  two  or  three  locules 
in  the  ovary. 

The  Sweet  Potato  is  of 
considerable  value  for  food 
and  is  quite  extensively 
grown  in  a  number  of 
states,  A  number  of 
plants  of  this  family  are 
weeds,  of  which  the  Morn- 
ing Glory  (Ipomoea),  Bind- 
weeds, and  Dodders  {Fig. 
439)  are  the  chief  ones. 
The  Morning  Glory  and 
Bindweeds  twine  around 
cultivated  plants,  cutting 
off  the  light  and  often 
breaking  them  down.  The 
Bindweeds  are  extremely  hard  to  eradicate  because  of  their 
spreading   roots   and    rootstocks   which    propagate   the   plants 


Fig.  441.  —  A  portion  of  a  Jiinson 
Weed  bearing  flowers  and  fruit.  Both 
sepals  and  petals  are  joined  most  of  their 
length. 


NIGHTSHADE   FAMILY 


491 


very  rapidly.  The  Dodders  are  parasitic  plants  and  do  much 
damage  in  Clover,  Alfalfa,  and  Flax  fields,  where  they  twine 
about  the  plants  and  grow  their  roots  into  their,  stems  and 
rob  them  of  their  food. 

Nightshade  Family  (Solonaceae) .  —  This  family  is  the  one  to 
which  the  Irish  Potato,  Tomato,  and  Tobacco  belong.  Some 
authors  give  the  number  of  species  as  about  1700.  Both  the 
five  sepals  and  five  petals 
are  more  or  less  joined 
{Fig.  UO).  The  stamens 
are  five  and  usually  inserted 
on  the  corolla.  The  Irish 
Potato  {Solarium  tuberosum) 
is  probably  the  most  im- 
portant plant  of  this  group 
and  Tobacco  {Nicotiana 
Tahacum)  next.  Some  years 
the  potato  crop  in  the 
United  States  is  more  than 
300,000,000  bushels.  New 
York  is  the  chief  potato 
growing  state,  although 
many  potatoes  are  grown  in 
Michigan,  Wisconsin,  and 
Pennsylvania. 

The  Tomato  {Ly coper si- 
cuni  esculentum) ,  when  first 
introduced  from  tropical 
America  as  an  ornamental 
plant,  was  considered  poison- 
ous, but  now  its  fruits  are 
important  vegetables. 

In  some  of  the  Southern  states,  as  Kentucky,  North  Caro- 
lina, and  Virginia,  Tobacco  is  one  of  the  leading  agricultural 
products,  while  in  many  other  states  it  is  grown  in  considerable 
quantities.  Some  other  cultivated  plants  of  this  family  are  the 
E^g  Plant,  Cayenne  Pepper,  Petunia,  and  Belladonna. 

To  this  family  belong  a  number  of  weeds,  some  of  which  arc 
quite  troublesome.  The  Black  Nightshade  {Solarium  nigrum) 
and  Jimson  Weed  {Datura  Stramonium)  {Fig.  440  ^J-re  common 


Fig.  442.  —  A  portion  of  the  Horse 
Nettle,  showing  flowers  and  fruits  and 
the  spiny  character  of  the  plant  (Xj). 
After  Dewey. 


492 


ANGIOSPERMS 


and    it    has   a  deep   rootstock, 
Another   troiililesonie    weed    of 


weeds  that  are  poisonous.      The  Horse  Nettle  (Solanum  caro- 
linense)  (Fig.  4^2)  is  troublesome  on  account  of  its  spiny  stems, 

which  is  difficult  to  eradicate, 
this  family  is  the  Buffalo  Bur 
(Solanum  rodratum)  (Fig. 
.'i-'iS),  which  has  spiny  fruits 
that  catch  into  the  wool  and 
hair  of  livestock. 

The  Madder  Family 
(Ruhiaceae).  —  This  is  one 
of  the  largest  families  of  the 
Dicotyledons.  There  are 
more  than  4000  species  be- 
longing to  this  family,  but 
the  majority  of  them  are 
tropical.  They  include 
herbs,  shrubs,  and  trees. 
Their  flowers  are  epigynous, 
and  the  stamens  and  lobes 
of  the  calyx  and  corolla 
are  the  same  in  number 
(usually  4-5). 

The  Coffee  tree  (Fig.  U4), 
which  is  grown  extensively 
in  Brazil,  Arabia,  and  Java, 
is  the  most  important  plant 
of  the  family.  The  fruit 
(Fig.  445)  of  the  Coffee  tree 
is  a  cherry-like  drupe  containing  two  seeds,  and  these  seeds  are 
the  coffee  of  commerce.  The  Cinchona  tree,  growing  wild  in 
the  Andes  and  cultivated  in  India,  furnishes  Cinchona  bark 
from  which  quinine  is  made. 

Gourd  Family  (Cucurbitaceae).  —  This  family  includes  the 
Gourds,  Pumpkins,  Squashes,  Melons,  and  Cucumbers.  The 
flowers  are  epigynous,  and  the  plants  are  monoecious  or  dioecious 
(Fig.  6).  The  stamens  are  usually  more  or  less  united.  In 
our  region  there  are  only  a  few  species  of  this  family  and  none 
of  much  importance  except  those  mentioned  above. 

Composite  Family  (Composite).  —  This  is  an  immense  fam- 
ily and  is  of  world-wide  distribution.     It  is  the  highest  group 


Fig.  443. —  a  plant  of  the  Buffalo 
Bur  bearing  flowers  and  fruits,  showing 
the  character  of  the  plant  ( X  yV)  ',  and  a 
single  flower,  showing  the  prickly  calyx 
and  gamopetalous  corolla.   After  Dewey. 


COMPOSITE   FAMILY 


493 


of  Dicotyledons.  The  most  conspicuous  charactor  of  the 
family  is  the  grouping  of  the  flowers  into  a  compact  head,  which 
is  surrounded  by  bracts 


forming  the  structure 
called  involucre  {Fig. 
446)'-  The  flowers  are 
epigynous,  the  corolla 
is  usually  tubular  or 
strap-shaped,  and  the 
five  stamens  are  in- 
serted on  the  corolla 
and  usually  have  their 
anthers  united  in  a  tube 
around  the  style.  The 
calyx  is  often  a  tuft  of 
hairs  (pappus) .  They 
have  developed  very 
effective  means  of  dis- 
seminating their  seeds. 
In  many,  as  the  Dande- 
lion and  Thistles  illus- 
trate, the  pappus  forms 
a  parachute-like  ar- 
rangement, which  enables  the  fruit  to  l^e  easily  transported  by 

the  wind.  In  others,  as  the 
B  u  r  d  o  c  k,  Cocklebur,  and 
Spanish  Needles  illustrate, 
the  fruits  have  hooks  or 
spines,  which  catch  onto  pass- 
ing animals. 

Although  the  family  is  a 
large  one,  it  contains  only  a 
few  food  plants,  of  which  Let- 
tuce, Chicory,  Oyster  plant, 
the  Globe  Artichoke,  and 
Jerusalem  Artichoke  are  the 
chief  ones.  Some,  as  Arnica, 
Boneset,  Camomile,  DandoHon,  Tansy,  and  Wormwood,  are 
used  some  for  Medicine,  and  from  the  seeds  of  the  Sunflower  oil 
is  extracted. 


Fig.  444.  —  A  Coffee  tree  in  fruit. 
After  Lecomte. 


Fig.  445.  —  Flower,  fruit,  and  seeds 
of  the  Coffee.  At  the  left,  a  flower, 
and  at  the  right,  a  fruit  with  the  upper 
portion  of  the  ovary  removed  to  show 
the  two  seeds.     After  Karsten. 


494 


ANGIOSPERMS 


The  family  includes  a  number  of  ornamental  plants,  of  which 
the  Cornflower  (also  called  Bachelor's  Button),  Marguerite,  China 
Aster,  Chrysanthemum,  Cosmos,  Dahlia,  and  English  Daisy  are 
familiar  ones. 

In  number  of  weeds  which  it  includes  this  family  surpasses  all 
others.     About  one  hundred  plants  of  this  family  are  classed 


Fig.  446.  —  The  Marguer- 
ite, one  of  the  Composites, 
showing  the  flowers  grouped 
into  a  compact  head  and  sur- 
rounded by  an  involucre.  In 
this  composite  a  head  con- 
tains two  kinds  of  flowers  — 
ligulate  flowers,  one  of  which 
is  shown  at  the  left,  and  tu- 
bular flowers,  one  of  which  is 
shown  at  the  right  of  the 
heads.     After  Lecomte. 


Fig.  447.  —  Canada 
Thistle,  showing  a  horizontal 
root  and  an  aerial  stem  in 
flower  (X|),  and  also  show- 
ing single  fruits  or  achenes, 
one  of  which  is  shown  with- 
out pappus  and  shghtly  en- 
larged. 


as  weeds,  although  not  many  of  them  are  bad  weeds.  The 
Canada  Thistle  {Fig.  44-7)  is  probably  the  worst  weed  of,  the 
family.  It  spreads  rapidly  by  spreading  roots  or  rootstocks 
and  soon  takes  possession  of  pastures  and  meadows  and  gives 
considerable  trouble  in  cultivated  ground.  On  account  of  the 
spreading  underground  structures  which  propagate  readily  when 
cut  into  pieces,  the  plant  is  exceedingly  hard  to  eradicate.  Some 
of  the  other  Thistles  are  also  quite  troublesome  in  some  regions. 


GRASS   FAMILY 


495 


Some  other  well-known  weeds  of  the  family  are  the  Cockle- 
burs,  Ragweeds,  Ironweeds,  Spanish  Needles,  Wild  Lettuce, 
and  Beggar-ticks. 

Monocotyledons 

Among  Monocotjdedons  about  25,000  species  are  recognized, 
which  are  distributed  among  42  families.     They  are  less  than 
one-fourth   as   numerous   as  the   Dicotyledons.     As   previously 
stated,     Monocotyledons     differ     from 
Dicotyledons    in    having    flowers    with 
parts  usually  in  threes  or  sixes,  leaves 
with  parallel  veins  except  in  rare  cases, 
and  vascular  bundles  with  the  scattered 
arrangement.      The  Monocotyledons 
contain  a  few  famihes  of  economic  im- 
portance and  one  family  that  surpasses 
all    other    groups    of    Angiosperms    in 
number  of  valuable  food  plants. 

Cat-tail  Family  ( Typhaceac) .  —  This 
family  is  mentioned  because  it  includes 
the  simplest  of  the  Monocotyledons. 
They  are  aquatic  plants,  growing  in 
groups  in  swamps  and  wet  places.  Some 
get  as  high  as  one's  head,  and  in  the 
late  summer  and  fall,  when  their  in- 
florescences resembhng  a  cat's  tail  are 
well  formed,  they  are  conspicuous  plants 
{Fig.  44-8)  •  The  flowers  are  monoecious 
and  have  neither  caljrx  nor  corolla  {Fig. 
449)-  The  pistil  is  composed  of  one 
carpel  containing  one  locule  and  only 
one  ovule.     The  staminate  flowers  are 

borne  at  the  top  and  the  pistillate  flowers  below  on  the  spike. 
The  pistil  is  supported  by  a  stalk  or  stipe  which  develops  hairs 
that  become  the  brown  down  of  the  fruit.  The  stamens  are  at- 
tached directly  to  the  axis  of  the  spike  and  are  intermixed  with 
hairs.  As  to  whether  the  simple  flowers  of  the  Cat-tails  are  primi- 
tive or  are  r(Hluced  forms  of  more  complex  flowers  is  not  known. 

Grass  Family  {Gramineae).  —  The  Grasses  constitute  one  of 
the  largest  families  of  Angiosperms  and  are  widely  distributed 


Fig.  448.  — The  com- 
mon Cat-tail  {Thypha 
latifolia),  showing  the 
terminal  spikes  of  flowers 
consisting  of  staminate 
flowers  above  and  pistil- 
late flowers  below  (X  |). 


496 


ANGIOSPERIMS 


over   the   earth.     The    many   valuable   plants,    such   as    Corn, 


Wheat,  Oats,  Barley,  Rye,  Ri( 


Fig.  449.  —  The  flowers  of  the  com- 
mon Cat -tail.  At  the  left,  a  staminate 
flower;  at  the  right,  a  pistiUate  flower. 


Millet,  Sugar  Cane,  Sorghum, 
Blue-Grass,  Timothy,  and 
others  that  are  included,  make 
the  Grass  family  the  most  im- 
portant family  of  Angiosperms. 
Except  the  Bamboos,  which 
are  shrubs  or  trees,  the  Grasses 
are  herbaceous.  Some  have 
unisexual  {Fig.  14,  16),  while 
others  have  bisexual  flowers 
{Fig.  18),  and  the  flowers  are 
commonly  arranged  in  spikes 
or  panicles.  Their  chief  char- 
acteristics are  that  their  essen- 


tial reproductive  organs  are  enclosed  in  bracts  and  that  they 
have  a  nut-like  fruit  called  a  grain  or  cariopsis. 


Fig.  450.  —  Sugar  Cane.     Aft(>r  Looomtc 


Besides  the  grains  upon  which  mankind  depends  so  much  for 
food  and  the  Sugar  Cane  {Fig.  450),  which  is  grown  extensively 


PALM    FAMILY 


497 


in  the  Southern  states,  West  Indies,  Hawaii,  and  Java  for  sugar 
and  molasses,  there  are  the  meadow  and  pasture  Grasses  which 
are  highly  important  to  man.  The  Grasses  most  valuable  for 
hay  are  Timothy  and  Redtop,  while  the  Kentucky  Blue  Grass, 
Bermuda  Grass,  Brome  Gra.ss,  Meadow  Fescue,  and  Rye  Grass 
are  some  of  the  Grasses  useful  for  pasture. 

A  large  number  of  the  Grasses  are  weeds,  of  which  the  Sand 
Bur,  Foxtails,  Chess,  Wild  Oats,  Quack  Grass,  and  Darnel  are 
some  of  the  worst  ones.  In  the  Southern  states  Johnson  Grass, 
although   useful  for  hay  and   pasture,   is  regarded  as  a  weed 


Fig.  451.  — a  group  of  Date  Palms 


because  it  spreads  so  rapidly  by  rootstocks  to  regions  where  it 
is  not  wanted  and  is  so  hard  to  eradicate.  Quack  Grass,  which 
spreads  by  rootstocks,  is  a  bad  weed  in  some  of  the  Northern 
states.  The  Wild  Oats  is  troublesome  in  grain  fields,  and  grain 
containing  the  seeds  of  Wild  Oats  in  considerable  quantities  is 
usually  docked.  The  seeds  of  Darnel  are  poisonous  and,  when 
ground  with  Wheat,  make  the  flour  unwholesome,  for  which 
reason  Darnel  is  a  bad  weed. 

Palm  Family  (Palmaceae).  —  This  is  about  the  only  family  of 
Monocotyledons  that  contains  trees.     Nearly  all  of  the  family 


498 


ANGIOSPERMS 


Fig.  452.  —  A  portion  of  a  Lily  in 
flower  and  also  a  single  flower,  show- 
ing the  perianth  consisting  of  six 
similar  parts.     After  Lecomte. 


are  tropical  or  subtropical,  but  they  arc  quite  extensively  grown 

in  greenhouses  everywhere.     The  flowers  have  three  sepals,  three 

petals,  three  to  six  stamens, 
and  a  pistil  commonly  of  three 
united  carpels.  The  flowers 
are  borne  on  a  spadix  and  en- 
closed in  a  spathe.  The  fruit 
is  sometimes  a  berry,  as  in  the 
Date,  or  nut-like,  as  in  the 
Coconut.  A  numl)er  of  the 
palms  are  valuable  plants  in 
the  region  where  they  grow. 
The  Date  Palm  {Fig.  451) 
yields  the  dates  of  the  market. 
The  Coconut  Palm  yields  the 
Coconuts  of  the  market  and  is 
probably  one  of  the  most  use- 
ful Palms  to  the  natives,  fur- 
nishing food,  clothing,  utensils  of  all  kinds,  building  materials, 

etc.     The  Sago  Palms  yield  Sago,  which  is  prepared  by  washing 

out  the  starch   from   the    stems.     A 

tree  15  years  old  will  sometimes  yield 

800  lbs.  of  starch.     The  Oil  Palm  of 

West  Africa  yields  a  fruit  from  which 

palm  oil  is  obtained. 
Lily  Family  (Liliaceae) .  ■ —  The  Lilies 

have  a  perianth  of  six  parts  and  six 

stamens  (Fig.  452).    The  pistil  usually 

consists  of  three  united  carpels.    Their 

flowers  are  often  showy,  and  many  of 

them,  as   the  true  Lilies,  Hyacinths, 

Star  of  Bethlehem,  Tuhp,  Day  Lily, 

and  Lily  of  the  Valley,  are  ornamental 

plants.     The    Onion    and   Asparagus 

are  common  articles  of  food.     Some, 

as  the  Aloe,  Smilax,  Colchicum,  and 

Veratrum,  yield  valuable   medicines. 

One,  called  New  Zealand  Flax,  is  a  valuable  fiber  plant.     In 

another   family    closely    related    to    the    Lily    family    are    the 

Agaves,  of  which  the   Century  Plant  {Fig.   453)   is  a  familiar 


Fig.  453.  —  A  Century 
Plant,  one  of  the  Agaves, 
showing  the  thick  leaves  and 
shape  of  the  plant  (XxV). 


ORCHID   FAMILY 


499 


one  in  our  region.  Tlie  Agaves  are  very  important  plants  in 
Mexico  where  the  natives  obtain  from  them  pulque,  a  fermented 
drink,  mescal,  a  distilled  drink  resembling  rum,  and  various 
fibers  as  sisal  hemp  and  henequen. 

In  connection   with   families  noted  for  fibers   and    not    pre- 
viously referred  to,  there  is  the  Linden  family  of  the  Dicotyle- 
dons from  which  Jute  is  obtained  and  the 
Banana-like  plant  of  the  Banana  family 
from  the  leaf  stalks  of  which  Manila  hemp 
is  obtained. 

Orchid  Family  (Orchidaceae) . — This 
family  includes  the  most  highly  developed 
Monocotyledons,  and  if  the  Monocoty- 
ledons are  higher  than  the  Dicotyledons, 
then  the  plants  of  this  family  are  the  most 
highly  developed  of  the  plant  kingdom, 
In  the  Orchids  the  flowers  are  highly 
specialized,  often  very  showy,  and  present 
interesting  mechanisms  to  secure  cross- 
pollination.  The  family  includes  numer- 
ous species  and  nearly  one-fourth  of  the 
Monocotyledons.  The  most  highly  spe- 
cialized ones  are  tropical  and  occur  only 
in  greenhouses  in  the  temperate  regions. 

The  flowers  are  epigynous  and  show  extreme  irregularity 
{Fig.  454)-  One  of  the  petals  called  the  Up  varies  much  in  shape 
and  is  usually  very  different  from  the  other  petals.  The  one 
or  two  stamens  join  with  the  style  of  the  pistil  to  form  the  column. 
In  most  cases  the  pollen  sticks  together,  forming  masses  called 
poUinia,  and  it  is  in  these  masses  that  the  pollen  is  carried  from 
one  flower  to  another  by  insects. 


Fig.  454.  —  A  flower  of 
an  Orchid  {Cypripedium), 
showing  the  irregularity 
among  the  parts  of  the 
perianth,  and  an  insect 
entering  the  pouch-Uke 
structure  of  the  corolla. 
After  Gibson. 


CHAPTER   XXT 

ECOLOGICAL   CLASSIFICATION    OF    PLANTS 

Nature  of  Ecology 

It  is  common  observation  that  certain  kinds  of  plants  live 
only  in  certain  places.  Thus  regions  distinct  in  type,  such  as 
ponds,  bogs,  shady  ravines,  dry  hillsides,  etc.,  have  distinct 
types  of  vegetation.  The  plants  in  ponds  and  bogs  arc  ad- 
justed to  much  water,  in  shady  ravines  to  shade  and  moist 
air,  and  on  dry  exposed  hillsides  the  plants  are  adjusted  to  hot 
sunshine  and  dry  soil.  Certain  kinds  of  plants  are  therefore 
adjusted  to  a  certain  environment  which  is  known  as  their 
habitat.  In  order  to  thrive,  a  plant  must  be  able  to  compete 
with  other  plants  and  endure  the  hardships  which  the  environ- 
ment imposes  upon  it.  It  must  be  adjusted  to  the  range  of 
temperature,  amount  of  light  and  moisture,  conditions  of  the 
soil,  surrounding  plants  and  animals,  etc.  Plant  Ecology  is 
the  science  which  treats  of  the  adjustments  and  distribution 
of  plants  in  relation  to  the  various  environmental  factors. 
Throughout  the  preceding  chapters  Ecology  has  been  touched 
upon  repeatedly,  for  the  adjustment  of  leaves  and  stems  to 
light,  the  storage  of  food  in  tubers  and  seeds  for  the  next  gen- 
eration, the  adjustments  of  flowers  to  various  kinds  of  pollina- 
tion, the  parasitic  and  saprophytic  habits,  the  adjustments  for 
living  in  the  water  or  air,  etc.,  really  belong  to  Ecology.  In 
the  classification  of  plants  phylogenetically,  which  is  emphasized 
in  the  previous  chapters  of  Part  II,  the  basis  of  classification  is 
kinship,  but  in  classifying  plants  ecologically  the  basis  is  ad- 
justment to  environment,  and  plants  varying  widely  in  their 
phylogenetic  relationships  occur  together  in  the  same  ecological 
class.  For  example,  Thallophytes,  Bryophytes,  Pteridophytes, 
and  Spermatophytes  occur  together  in  some  ecological  classes 
of  water  plants. 

Many  of  the  problems  of  Agriculture  have  to  do  with  the 
securing  of  strains  or  varieties  of  crop  plants  better  adjusted 

500 


WARMTH  501 

to  a  particular  environineiit.  Wc  are  constantl}^  striving  to 
find  the  Apples,  Pears,  and  other  fruits  best  adjusted  to  the 
environmental  factors  of  different  regions.  One  of  the  objects 
in  the  breeding  of  Citrous  fruits  has  been  to  procure  varieties 
less  sensitive  to  cold,  so  that  Citrous  fruits  can  be  grown  farther 
north  and  consequently  over  a  larger  area.  Much  time  and 
energy  has  been  spent  in  obtaining  strains  of  Cotton  resistant 
to  the  insect  pests  and  other  unfavorable  environmental  factors 
of  the  Southern  states.  In  the  Northern  states,  where  the 
growing  season  is  short,  one  of  the  problems  in  connection  with 
the  raising  of  Corn  is  to  secure  varieties  that  can  mature  before 
frost.  The  securing  of  drought  resistant  plants  for  dry  regions, 
of  plants  resistant  to  the  diseases  prevalent  in  the  different 
agricultural  regions,  of  pasture  Grasses  best  adapted  to  a  given 
region,  of  trees  adapted  to  grow  in  a  given  region  for  shade  or  on 
a  given  area  that  is  to  be  reforested  are  some  of  the  many  other 
agricultural  prol)lems  that  have  to  do  with  adjustment  of  plants 
to  their  environment  and  hence  are  ecological. 


Ecological  Factors 

The  various  environmental  features  to  which  plants  and  ani- 
mals must  adjust  themselves  are  called  ecological  factors.  The 
chief  ecological  factors  are  water,  heat,  light,  soil,  wind,  and 
associated  plants  or  animals. 

Water.  —  This  is  one  of  the  most  important  ecological  factors. 
The  amount  of  water  to  which  various  plants  are  adj  usted  varies 
from  complete  submergence  to  perpetual  drought.  Most  Algae 
live  completely  submerged  in  water,  while  Cacti  are  adjusted 
to  the  drought  of  deserts.  Most  crop  plants  require  a  medium 
amount  of  water  in  the  soil,  and  an  excess  or  lack  of  water  re- 
tards their  growth.  But  among  crop  plants  there  is  also  much 
variation  in  the  amount  of  water  necessary  for  living.  For 
example,  the  Sorghums  are  more  resistant  to  drought  than 
Corn,  while  some  varieties  of  Rice  require  flooding. 

Warmth.  —  All  kinds  of  plants  are  adjusted  to  certain  ranges 
of  temperature.  For  example,  Wheat  and  Oats  require  less 
warmth  than  Corn,  and  hence  can  be  grown  farther  north. 
There  are  great  zones  of  plants  corresponding  to  the  great 
zones  of  temperature.     Thus  the  arctic,  temperate,  and  tropi- 


502  ECOLOGICAL  CLASSIFICATION   OF  PLANTS 

cal  zones  are  distinguished  by  their  kinds  of  plants  as  well  as 
by  their  difference  in  temperature.  When  the  temperature  is 
extremely  low,  as  in  the  polar  ice  regions,  or  extremely  hot,  as 
in  some  deserts,  very  few  or  no  plants  at  all  are  able  to  live. 
Even  on  the  same  area,  as  in  a  woods  or  a  field,  if  the  plants  are 
not  disturbed,  one  can  observe  the  effect  of  the  heat  factor  in 
the  succession  of  plants  through  the  growing  season,  the  spring 
plants  being  very  different  from  the  summer  and  autumn 
plants.  To  secure  crop  plants  adapted  to  the  temperatures  of 
the  different  agricultural  regions  is  also  one  of  the  pro])lems  of 
Agriculture. 

Light.  —  Not  all  plants  in  an  association  can  receive  the  same 
amount  of  light,  and  some  plants  are  so  adjusted  that  they  can  live 
in  the  shade.  They  are  known  as  shade  plants,  and  the  Ferns, 
common  in  the  woods,  are  examples  of  such  plants.  But  even 
in  an  association  of  herbaceous  plants,  as  in  a  field  of  weeds, 
many  small  plants  grow  among  and  in  the  shade  of  the  taller 
ones.  Some  plants,  like  the  Pumpkins  and  Melons  which  grow 
well  along  with  Corn,  have  a  ver}^  large  leaf  surface  which  may 
compensate  for  the  lack  of  light.  Plants  climb  other  plants  or 
walls,  grow  tall  erect  stems,  and  adjust  themselves  to  neigh- 
boring plants  in  various  other  ways  in  order  to  obtain  sufficient 
light. 

Soil.  —  The  soil  in  regard  to  its  chemical  and  physical  prop- 
erties determines  largely  the  kinds  of  plants  that  can  grow  in  a 
given  region.  Thus  the  plants  on  a  sandy  beach  or  sand  dune 
differ  from  those  on  a  clay  or  loam  soil.  The  chemical  elements 
of  a  soil  and  its  power  to  retain  water  both  have  a  determining 
effect  upon  the  growth  of  plants.  Some  plants,  like  Alfalfa  and 
some  of  the  Clovers,  are  more  sensitive  than  the  grains  to  acids 
in  the  soil.  Some  weeds,  like  the  Sheep  Sorrel,  grow  best  in  an 
acid  soil.  Some  plants  require  more  potash,  nitrates,  or  some 
other  element  than  other  plants.  Even  water  plants  are  some- 
what dependent  upon  the  soil,  for  the  minerals  in  a  pond  or 
lake  are  carried  in  from  the  soil.  One  of  the  chief  problems  of 
Agriculture  consists  in  putting  the  soil  in  a  suitable  condition 
for  plants  and  in  choosing  plants  adapted  to  the  different  types 
of  soil. 

Wind.  —  The  wind  tends  to  dry  out  plants  by  increasing  their 
transpiration,  while  at  the  same  time  it  is  an  important  agent 


ASSOCIATED   PLANTS   AND   ANIMALS  503 

in  pollination  and  dissemination  of  fruits  and  seeds.  In 
regions  where  there  are  strong  prevailing  winds  only  such  plants 
as  are  adapted  to  regulate  transpiration  can  grow.  Most  of  our 
early  flowering  plants,  as  the  Pines,  Oaks,  Beeches,  and  Poplars, 
are  pollinated  by  the  wind,  and  some  of  our  crop  plants,  as  Corn 
illustrates,  depend  largely  upon  the  wind  for  pollination.  For 
the  wide  dissemination  of  the  fruits  and  seeds  of  many  of  the 
common  weeds  and  of  some  cultivated  plants,  and  also  for  the 
spreading  of  some  fungous  diseases  the  wind  is  responsible. 

Associated  Plants  and  Animals.  —  A  plant  must  compete 
with  surrounding  plants  and  often  with  animals  for  existence. 
It  is  common  observation  that  most  crop  plants  will  not  do  well 
under  the  shade  of  trees.  The  trees  cut  off  the  light  and  make 
the  soil  too  dry  for  the  crop  plants.  On  the  other  hand,  there 
are  plants  which  require  shade  and  hence  grow  best  in  the  woods. 
In  some  cases  plants  are  benefited  while  in  other  cases  they  are 
injured  through  the  association  of  their  roots  with  the  roots  of 
other  kinds  of  plants.  For  example,  when  Corn  and  Clover 
are  grown  together,  experiments  indicate  that  Corn  does  better 
than  when  it  is  grown  alone.  One  experimenter  grew  Oats, 
Barley,  Buckwheat,  Wheat,  and  Flax  in  pots  with  and  without 
the  underground  shoots  of  Canada  Thistle  and  found  all  except 
Buckwheat  to  grow  better  with  the  Canada  Thistle  than  alone. 
He  repeated  the  experiment,  using  a  young  Elm  tree  instead 
of  the  Canada  Thistle,  and  found  that  all  grew  more  poorly  Avith 
the  Elm  tree  than  alone.  In  Jutland  it  is  found  that  Spruce 
trees  grow  well  on  waste  areas  if  their  roots  can  associate  with 
those  of  the  Mountain  Pine.  If  there  are  no  Mountain  Pines 
present,  the  Spruces  will  not  grow.  If  the  Pines  are  present 
but  are  cut  before  the  Spruces  get  well  started,  the  Spruces  die 
or  make  a  poor  growth.  No  doubt  much  injury  to  crops  caused 
by  weeds  is  due  to  the  antagonistic  effects  of  their  root  systems. 
The  association  of  certain  kinds  of  nitrogen-fixing  Bacteria 
with  the  roots  of  legumes  and  of  parasitic  plants  with  their 
hosts  are  familiar  examples  of  a  very  intimate  relation  of  the 
life  processes  of  one  plant  with  those  of  another.  In  competing 
for  light,  as  previously  pointed  out,  plants  must  adjust  themselves 
to  each  other  in  various  ways.  Climbing  plants,  in  securing  a 
better  position  in  reference  to  light  for  themselves,  frequently 
injure   the    plant   which   they   climb.     For   example.    Morning 


504  ECOLOGICAL  CLASSIFICATION   OF  PLANTS 

Glories  and  Bindweeds  cut  off  the  light  and  break  plants  down, 
and  Grape  vines  often  injure  the  trees  over  which  they  spread. 

In  a  number  of  ways  plants  are  adjusted  to  animals.  The 
presence  of  thorns,  stinging  hairs,  and  bitter  juices  may  pro- 
tect plants  against  destruction  by  animals.  Insect  pollina- 
tion is  a  notable  example  of  the  dependence  of  plants  upon  ani- 
mals. The  flowers  of  some  plants  are  so  adjusted  that  they 
require  insects  and  often  certain  species  of  insects  to  pollina'te 
them.  Thus  bees  are  required  to  cross-pollinate  Red  Clover, 
and  Sweet  Clover  and  Alfalfa,  although  they  do  not  require 
cross-pollination,  require  insects  which  can  trip  their  flowers, 
so  that  the  pollen  can  get  on  the  stigma.  It  is  also  recognized 
that  bees  are  essential  to  good  pollination  in  orchards.  Orchids 
and  Yuccas  are  two  of  the  most  notable  examples  of  plants 
which  have  flowers  so  constructed  that  only  certain  types  of 
insects  can  pollinate  them.  In  such  cases  it  is  obvious  that 
propagation  by  seeds  depends  upon  the  presence  of  the  insects 
which  are  required  to  pollinate  the  flowers.  For  securing  the 
dissemination  of  their  seeds,  plants  are  adjusted  to  animals  in 
a  number  of  ways,  but  chiefly  by  developing  hooked  or  spiny 
fruits  or  seeds  which   cling  to  the   coats   of   animals. 

The  above  factors  with  minor  ones  largely  determine  the 
modifications  and  distribution  of  plants.  These  factors  work 
together  and  not  singly,  and  the  combinations  of  factors  are 
numerous.  According  to  their  adjustment  to  the  ecological 
factors,  plants  fall  into  groups  or  classes  known  as  societies. 
Thus  all  plants  adjusted  to  a  water  habitat  belong  to  a 
hydrophytic  society  and  are  called  Hydrophytes,  while  those 
adjusted  to  a  drought  habitat  belong  to  a  xerophytic  society 
and  are  known  as  Xerophytes. 


Ecological  Societies 

Since  the  ecological  factors  and  their  combinations  vary 
widely,  there  are  many  different  habitats  and  hence  many  eco- 
logical societies.  With  reference  to  the  water  factor  plants  are 
grouped  into  Hydrophytic,  Mesophytic,  and  Xerophytic  soci- 
eties. 

Hydrophytic  Societies.  —  These  are  the  societies  of  water 
plants  called   Hydrophytes  and  include  plants  which  live  sub- 


HYDROPHYTIC   SOCIETIES  505 

merged,  staiulin-i  in  tlie  water,  or  floating  on  the  surface  of  the 
water.  As  previousl}^  stated,  the}^  include  plants  from  various 
phylogenetic  groups.  Many  are  Thallophytes,  as  the  Algae 
illustrate,  while  some,  like  the  Pond  Lihes,  Duckweeds,  Pond- 
weeds,  Eelgrass,  and  others,  are  Angiosperms.  Not  only  repre- 
sentatives of  the  lowest  and  highest  divisions  of  the  Plant 
Kingdom,  but  also  some  Bryophytes  and  Pteridophytes  are 
included  in  these  societies. 

The  hydrophytes  are  adapted  in  various  ways  to  living  in  the 
water.  In  the  Algae  the  unicellular  and  filamentous  bodies  with 
all  cells  thin-walled  afford  the  maximum  amount  of  surface  for 
absorbing  gases  and  minerals  from  the  water  and  for  absorbing 
the  light  that  reaches  them.  The  more  massive  Algae  are  com- 
monly so  anchored  that  they  are  aerated  through  wave  action, 
and  many  are  provided  with  floats  or  air  chambers  wherel^y  they 
float  near  the  surface  where  there  are  more  gases  and  light  than 
at  greater  depths.  The  more  complex  Hydrophytes,  such  as 
the  Seed  Plants,  that  live  chiefly  submerged  in  the  water  have  a 
thin-walled  epidermis,  so  that  all  parts  of  the  plant  can  absorb, 
and  water-conducting  tissues  are  feebly  developed.  Since  they 
depend  upon  the  buoyant  power  of  the  water  for  support,  the 
root  system  is  commonly  reduced  or  even  wanting,  and  their 
mechanical  tissues  are  not  so  well  developed  as  those  of  Seed 
Plants  that  live  on  land.  Usually  such  plants  collapse  when 
taken  out  of  the  water.  Some,  like  the  Pond  Lihes,  raise  their 
leaves  to  the  surface  of  the  water  where  they  receive  good 
light,  while  others,  as  the  Pondweeds  and  Eelgrass  illustrate,  are 
wholly  submerged  and  are  able  to  get  along  with  the  little  light 
that  reaches  them.  The  submerged  forms  even  bear  their  flow- 
ers under  water.  Among  the  Hydrophytic  societies  there  are 
the  free-swimming,  pondweed,  and  swamp  societies. 

The  free-swimming  societies  are  made  up  of  such  plants  as 
the  Diatoms,  Algae,  Duckweeds,  and  other  plants  which  float 
in  stagnant  or  slow-moving  water. 

In  the  pondweed  societies  the  plants  are  anchored,  but  their 
bodies  are  sul^merged  or  floating  {Fig.  455).  To  this  society 
belong  the  Water  Lilies,  Pondweeds,  Water  Ferns,  Marine  Algae, 
some  fresh-water  Algae,  and  some  species  of  Mosses. 

Swamp  societies  consist  of  water  plants  which  have  leaf-]i(\u'- 
ing  stems  reaching  above  the  surface  of  the  water.     Some  typical 


506 


ECOLOGICAL  CLASSIFICATION   OF  PLANTS 


plants  of  swamp  societies  are  the  Sagittarias,  Bulrushes,  Cat-tails, 
Rushes,  Sedges,  and  Reedgrasses,  which  form  fringes  around 
ponds  and  lakes  {Figs.  456  and  457).  Some  trees,  such  as  Wil- 
lows, Poplars,  Birches,  and  Alders,  are  common  in  swamp  societies. 
In  a  swamp  of  the  bog  type,  Spliagnum  Moss,  Orchids,  and  some 
trees,  such  as  the  Tamarack,  Pine,  and  Hemlock,  are  character- 
istic plants. 

Aside  from  Rice,  which  is  a  Hydrophyte  during  a  part  of  its 


Fig.  455.  —  A  pond  in  which  are  growing  Water  Lilies,  plants  tj-pical  of  a 
Pond-weed  society.     After  C.  M.  King. 


development,  the  hydrophytic  societies  are  not  noted  for  plants 
important  economically. 

Mesophytic  Societies.  —  The  mesophytic  societies  comprise 
the  common  vegetation.  They  require  a  medium  amount  of 
moisture  and  a  fertile  soil.  To  these  societies  belong  our  culti- 
vated plants,  weeds,  and  deciduous  forests.  The  mesophytic 
condition  is  the  arable  condition  and  is  the  normal  or  optimum 
condition  for  plants.  If  a  hydrophytic  area  is  to  be  cultivated, 
it  must  be  drained  and  made  mesophytic. 


MESOPHYTIC   SOCIETIES 


507 


Fig.  456.  —  A  swamp  society  consisting  chiefly  of  Sagittarias  and  Sedges. 
After  C.  M.  King. 


Fig.  457.  —  A  swamp  society  in  which  Cat -tails  are  dominant. 
After  CM.  King. 


508  ECOLOGICAL  CLASSIFICATION  OF  PLANTS 

Mesophytes,  in  contrast  to  Hydrophytes,  are  exposed  much 
more  to  the  chying  effect  of  the  air  and  consequently  are  better 
protected  against  transpiration.  They  need  better  root  systems 
for  absorption  and  anchorage  and  also  have  better  developed 
conductive  and  mechanical  tissues.  There  are  many  types  of 
mesophytic  societies. 

Meadows  and  prairies  are  mesoph\'tic  societies  in  which  trees 
are  absent,  and  the  dominant  plants  are,  therefore,  grasses  and 
other  herbaceous   plants   {Fig.  4^8).     The  most  important   of 


Fig.  458.  —  A  prairie,  a  mesophytic  society  in  which  trees  are  absent. 

the  woody  mesophytic  societies  are  the  deciduous  forests  com- 
posed of  Maples,  Beeches,  Oaks,  Tulips,  Elms,  Walnuts,  and 
other  valuable  trees  (Fig.  459).  In  such  forests  grow  also  char- 
acteristic societies  of  herbaceous  plants.  The  thicket,  composed 
of  small  woody  plants,  such  as  Willow^s,  Birches,  Alders,  Hazel 
bushes,  etc.,  is  another  woody  mesophytic  society.  The  most 
remarkable  of  the  mesophytic  societies  are  the  rainy  tropical 
forests,  where,  due  to  a  heavy  rainfall  and  great  heat,  vegeta- 
tion reaches  its  climax,  and  gigantic  jungles  are  developed,  com- 
posed of  trees  of  various  heights,  shrubs  of  all  sizes,  tall  and 
low  herbs,  all  bound  together  in  a  great  tangle  by  vines  and 
covered  by  numerous  epiphytes. 


XEROPTTYTTC   SOCIETIES 


509 


Fig.  459.  —  A  deciduous  forest,  a  mesophytic  society  consisting  of  Bass- 
wood,  Birches,  Elms,  Maples,  and  Oaks,  under  which  grow  many  herbaceous 
plants.     After  C.  M.  King. 

Xerophytic  Societies.  —  These  are  the  societies  adapted  to 
drouj2:ht.  Ainoiif!;  xerophytic  plants  there  are  various  adapta- 
tions to  drought,  such  as  sunken  stoniata,  hairy  epidermis,  re- 
duction of  leaf  surface,  deep  tap-roots,  reservoirs  within  the 
leaves  or  other  parts  of  the  plant  for  holding  water,  edgewise 
position  or  rolling  of  leaves,  bridging  over  the  period  of  drought 
in  the  form  of  seeds  or  subterranean  structures,  etc. 

Among  the  xerophytic  societies  are  the  rock  societies,  composed 
chiefly  of  Lichens  {Fig.  JfiO)  and  Mosses  which  grow  on  dry  jind 
exposed  rocks;  desert  and  dry  plain  societies  {Fig.  461)  where  such 
plants  as  Cacti,  Sage  Brush,  Agaves,  and  Yuccas  dominate;  xero- 
phytic thickets,  composed  of  a  dense  mass  of  bushes  and  repre- 
sented by  the  chaparral  of  the  Southwest;  and  the  xerophytic 
forests,  in  which  Pines,  Spruces,  and  Firs,  adapted  to  mountain 
slopes  and  gravel  ridges,  occur. 


510 


E('OLOOICAL   CLASSIFICATION  OF  PLANTS 


In  Asia,  Africa,  and  North  America,  there  is  much  land  that 
is  xerophytic.     Much  of  the  Southwestern  part  of  the  United 
States  is  xerophytic.     One  of  the  important  problems  in  Agri- 
culture is  to  bring  xerophytic 


C^C-r^f^^^^^^:^ 


^'£'^':.  > 


areas  into  cultivation.  This 
may  be  done  by  making  these 
areas  mesophytic  through  irri- 
gation or  by  securing  crop 
plants  through  selection  or 
breeding  that  are  drought  re- 
sistant, that  is,  able  to  grow- 
under  xerophytic  conditions. 


Fig.  460.  —  A  xerophytic  society, 
consisting  of  Lichens  growing  on  a 
bare  rock.     After  Bailey. 


Plant  Succession 

One  society  of  plants  com- 
monly prepares  the  way  for 
another.  For  example,  the  Lichens  and  Mosses,  growing  on  bare 
rocks,  disintegrate  the  rocks  and  form  soil  in  which  other  plants 
can  get  a  start  {Fig.  460).    Ponds  and  lakes  are  gradually,  filled  up 


Fig.  461  —  A  desert  xerophytic  society  consisting  chiefly  of  Sage  Brush 
and  Yuccas.     After  R.  G.  Kirby. 


through  the  growth  of  pond  societies  until  they  are  transformed 
into  swamps,  in  which  the  Pond  Lihes,  Pondweeds,  Eelgrass, 
and  other  representatives  of  pond  societies  are  replaced  by 
Rushes,  Sedges,  Sagittaraes,  Cat-tails,  Reeds,  True  Flags,  and 


PLANT  HUCCESSIOxN 


511 


other  representatives  of  swamp  societies.  Through  the  growth 
of  the  swamp  societies,  the  swamp  is  finally  so  filled  up  that  it 
is  transformed  into  a  mesophytic  area,  and  the  plants  of  the 
swamp  societies  are  succeeded  by  Mesophytes  {Figs.  4(^2  and 
Jf.63).  It  is  obvious  that  the  hydrophytic  societies  have  been 
exceedingly  important  factors  in  transforming  lakes,  ponds,  and 
old  river  beds  into  tillable  land,  and  the  fertile  soil  of   such 


Fig.  462.  —  A  succession  of  plant  societies,  showing  transition  from  hj'dro- 
phytic  to  mesophytic  societies.  The  successive  societies  are  as  follows: 
Pond  Lily  Society,  Sedge  Society  at  the  margin  of  the  pond  and  grading  into 
a  Swamp  Grass  Society  further  back,  a  shrub  society  still  further  back,  and 
finally  in  the  background  a  mesophytic  forest  society.  From  Coulter,  photo, 
by  Lewis. 


areas  is  largely  due  to  the  humus  added  through  the  decaj^  of 
the  hydrophytic  societies.  On  sand  dunes,  beaches,  ground 
cleared  and  allowed  to  grow  up  again,  and  most  everywhere  one 
can  observe  plant  succession.  On  sand  dunes  around  the  Great 
Lakes,  for  example,  Poplars  are  succeeded  by  Pines,  which  are 
in  turn  succeeded  by  Oaks  and  other  deciduous  trees. 

Studies  of  successions  and  societies  give  us  very  useful   in- 


512 


ECOLOCilCAL  CLASSIFICATION  OF   PLANTS 


formation  a.s  to  what  plants  can  be  successfully  grown  on  a 
given  ai-ea.  There  are  instances,  as  in  case  of  some  of  the  wild 
lands  of  tiie  West,  where  a  study  of  the  societies  of  wild  plants 
has  suggested  the  kind  of  crop  plants  best  adapted  to  the  condi- 
tions. It  is  quite  probable  that  more  extended  studies  in  Ecol- 
ogy in  connection  \\ith  soil   analyses  will   reveal  such  a  close 


Fig.  463.  —  A  lake  which  is  being  rapidly  hlled  up  by  the  accumulation 
of  vegetable  matter.  Swamp  societies  consisting  of  clumps  of  Rushes,  Sedges, 
and  Sagittarias  are  most  conspicuous  about  the  water.  Further  back  are 
swamp  Grasses  grading  into  mesophytic  Grasses,  and  finally  on  the  ridge, 
as  shown  by  the  corn  field  and  trees,  a  typical  mesophytic  condition  prevails. 
After  C.  M.  King. 


association  of  plant  societies  and  the  chemical  and  physical 
characteristics  of  soils  that  the  chemical  and  physical  differ- 
ences of  soils  on  different  farms  or  in  different  parts  of  the  same 
farm  may  be  quite  accurately  judged  by  observing  the  societies 
of  weeds  and  other  wild  plants.  In  reforesting  a  given  area  it 
is  very  essential  to  take  into  consideration  the  plant  societies 
adapted  to  the  region.  For  example,  it  would  be  unwise  to  plant 
Pines  on  bare  sand  dunes,  or  Maples  where  Black  Oaks,  which 
grow  in  much  drier  situations  than  Maples,  prevail. 


CHAPTER   XXII 
EVOLUTION 

Meaning   and   Theories   of   Evolution 

Meaning  of  Evolution.  —  Throughout  the  preceding  discus- 
sion of  plant  groups  evolution  was  assumed.  It  was  assumed 
that  the  more  complex  forms  of  plants  have  come  from  the 
simpler  ones  by  gradual  changes  which  involved  the  modifi- 
cation of  structures  and  the  introduction  of  new  structures. 
It  was  also  assumed  that  some  simpler  plants  are  reduced  forms 
of  more  complex  plants.  Hence  evolution,  which  is  usually 
forward,  that  is,  leading  to  more  advanced  forms,  may  be 
backward,  leading  to  simpler  forms.  According  to  the  theory 
of  evolution,  the  organisms  which  first  inhabited  the  earth  were 
extremely  simple,  and  the  various  forms  of  plants  and  animals 
which  we  now  have  are  their  modified  descendants. 

The  evidences  of  evolution  in  both  plants  and  animals  are 
obtained  by  studying  the  structure,  development,  and  behavior 
of  living  forms,  and  the  structure  of  ancient  forms  preserved  as 
fossils.  Evolution  takes  place  too  slowly  to  be  observed  or 
demonstrated,  and  hence  our  conclusions  about  it  are  only 
inferences. 

There  are  two  kinds  of  evolution  —  organic  and  inorganic. 
Organic  evolution  is  confined  to  living  things  including  animals 
as  well  as  plants.  However,  inorganic  things  are  also  constantly 
changing,  and  hence  evolution  applies  to  all  nature  and  not 
merel}^  to  living  things.  The  physical  features  of  the  earth  are 
constantly  changing.  Mountains  are  being  worn  down,  while 
there  are  other  regions  where  the  land  is  becoming  more  ele- 
vated. Areas  that  are  now  land  were  seas  or  a  part  of  the  ocean 
at  one  time.  A  study  of  fossil  plants  shows  that  climates  have 
changed,  so  that  regions  once  tropical  now  have  a  temperate 
or  arctic  climate.  Rivers  and  valleys  are  constantly  changing 
and  thus  altering  the  landscape.  Also  the  planets,  like  our  own 
earth,  and  even  the  stars,  arc  3hanging  externally  and  internally. 

513 


514  EVOLUTION 

Again,  the  phenomenon  of  radioactivity  teaches  us  that  the 
atoms  composing  one  chemical  substance  are  transformed  into 
those  of  another,  and  thus  chemical  substances  are  built  up  by 
the  process  of  evolution.  There  seems  to  be  but  one  thing  that 
is  constant  and  that  is  constant  change. 

Evolution  and  the  Doctrine  of  Special  Creation.  —  The  theory 
of  evolution  is  directly  opposed  to  the  "  Doctrine  of  Special 
Creation."  The  "  Doctrine  of  Special  Creation  "  is  based  upon 
a  literal  interpretation  of  the  account  of  creation  given  in  the 
Bible  and,  therefore,  assumes  that  all  things  were  created  at  the 
beginning  of  the  world.  According  to  the  "  Doctrine  of  Special 
Creation  "  plants,  animals,  mountains,  oceans,  planets,  and  stars 
were  created  in  the  beginning  by  the  Creator  and  have  remained 
constant  in  all  fundamental  features  until  the  present  time. 
This  means  that  Angiosperms  and  all  other  groups  of  plants, 
however  simple  or  complex,  did  not  come  from  simpler  forms, 
but  were  made  in  the  beginning  and,  therefore,  have  been  in 
existence  practically  ever  since  the  world  began.  It  is  thus 
seen  that  the  "  Doctrine  of  Special  Creation  "  is  not  at  all  in 
harmony  with  the  theory  of  evolution,  for  the  latter  theory  as- 
sumes that  in  the  early  history  of  the  world  there  were  only  very 
simple  organisms,  and  that  from  them  through  changes  involving 
many  millions  of  years  the  complex  forms  have  been  derived. 

Although  a  few  believed  in  evolution  even  as  far  back  as  the 
Greek  philosophers,  the  "  Doctrine  of  Special  Creation  "  pre- 
vailed until  comparatively  a  short  time  ago.  During  the  last 
150  years,  the  theory  of  evolution  has  gradually  gained  favor, 
and,  since  Charles  Darwin's  time,  it  has  gained  such  supremacy 
that  it  is  now  a  fundamental  conception  not  only  in  botany 
and  zoology  but  also  in  such  subjects  as  history,  philosophy, 
sociology,  and  theology. 

Theories  in  Regard  to  Evolution.  —  The  most  difficult  thing 
about  evolution  is  its  explanation.  One  can  trace  connections 
between  the  higher  and  lower  forms,  but  to  explain  just  in  what 
way  one  form  arose  from  another  is  not  at  all  easy.  Scientists 
are  all  convinced  of  the  reality  of  evolution,  but  the  forces  or 
factors  which  bring  about  evolution  are  still  under  discussion. 

Earliest  Theories.  —  Perhaps  the  oldest  explanation  of  evo- 
lution was  that  of  Erasmus  Darwin,  Goethe  of  Germany,  and  a 
few  others  of  their  time.     According  to  the  explanation  of  these 


LAMARCK'S  EXPLANATION  515 

early  scientists,  plants  and  animals  are  changed  by  their  en- 
vironment, and  these  changes  are  then  inherited  by  the  offspring 
and  retained  as  long  as  the  environment  remains  constant.  For 
example,  according  to  this  explanation,  a  plant,  originally  smooth 
but  having  been  induced  to  become  hairy  through  exposure  to 
a  drier  chmate,  will  impart  the  hairy  feature  to  the  offspring, 
which  will  maintain  the  modification  until  the  environment  so 
changes  that  hairiness  is  lost.  In  this  way  the  origin  of  new  char- 
acters and  new  species  was  explained.  This  assumes  of  course 
that  a  change  in  any  part  of  the  plant  is  recorded  in  the  sperms 
and  eggs  of  the  plant  and,  therefore,  transmitted  to  the  progeny. 

Lamarck's  Explanation.  —  Lamarck,  a  noted  French  natur- 
alist whose  views  were  published  first  in  1801  and  in  an  en- 
larged form  in  1809,  offered  the  explanation  which  he  called 
''  Appetency,"  meaning  desire.  His  explanation  was  based  upon 
the  observation  that  the  organs  of  men  and  other  animals  are 
enlarged  and  strengthened  by  use  and  particularly  by  con- 
scious use.  For  example,  it  is  common  observation  that  one's 
muscles  enlarge  and  become  stronger  with  proper  use,  while, 
on  the  other  hand,  with  lack  of  use  they  decrease  in  size  and 
strength.  Long  continued  disuse  may  even  result  in  the  loss 
of  an  organ. 

There  are  three  ideas  involved  in  Lamarck's  explanation. 
First,  his  explanation  assumes  that  the  environment  of  animals 
and  plants  has  been  constantly  changing,  so  that  they  have  been 
constantly  subjected  to  changes  in  temperature,  moisture,  light, 
nutrition,  etc.  Second,  he  believed  that  all  living  things  have 
come  from  preexisting  forms  as  a  result  of  changes  which  were 
repponses  to  changes  in  the  environment  that  made  a  new  mode 
of  life  necessary.  Thus  the  neck  of  the  giraffe  lengthened  as 
a  result  of  the  animal's  effort  to  reach  leaves  on  high  branches. 
Also  the  form  of  the  body  of  reptiles,  such  as  snakes,  which 
glide  over  the  ground  and  conceal  themselves  in  the  Grass,  is 
due  to  the  mode  of  life  which  these  animals  have  adopted.  By 
repeated  efforts  of  the  animal  to  elongate  in  order  to  pass 
through  small  spaces,  the  body  became  extremely  elongated  and 
very  narrow,  and  since  long  legs  would  raise  the  body  too  high 
from  the  ground  and  short  legs  would  not  move  them  rapidly 
enough,  legs,  vestiges  of  which  are  still  in  their  plan  of  organiza- 
tion, were  finally  lost  and  another  means  of  transportation  was 


516  EVOLUTION 

evolved.  Although  Lamarck's  explanation,  as  based  upon  the 
use  and  disuse  of  organs,  appHes  particularly  to  animals,  he  also 
offered  an  explanation  for  evolution  in  plants.  In  case  of  plants, 
in  which  there  is  no  conscious  effort  as  in  animals,  he  assumed 
that  changes  in  the  environment  affect  the  body  of  a  plant 
directly  and  induce  modifications  which  may  become  sufficiently 
pronounced  to  ^  characterize  a  new  species.  Since  species  are 
the  units  of  all  other  groupings  of  plants  or  animals,  the  origin 
of  new  species  results  in  the  origin  of  new  genera,  new  families, 
new  orders,  and  so  on.  It  is,  therefore,  obvious  that  accounting 
for  the  origin  of  species  accounts  for  the  origin  of  all  those 
differences  upon  which  the  various  groupings  of  organisms  are 
based. 

Third,  Lamarck  believed  that  whatever  changes  a  plant  or 
animal  made  in  the  form,  structure,  or  function  of  its  body 
were  inherited  by  the  offspring.  To  the  succeeding  generation 
each  generation  transmits  what  it  inherited  and  whatever  addi- 
tional modifications  it  may  take  on.  In  this  way  modifications 
which  are  only  slight  at  first  may  become  more  pronounced  in 
succeeding  generations,  if  the  conditions  remain  constant,  until 
finally  plants  or  animals  so  different  from  their  ancestors  as  to 
form  new  species  may  arise.  He  did  not  claim  that  all  individ- 
uals taking  on  new  modifications  survive  but  only  those  pos- 
sessing changes  that  fit  them  most  perfectly  to  their  environ- 
ment. 

Lamarck's  explanation  is  unsatisfactory  in  a  number  of  ways. 
In  the  first  place  both  observations  and  experiments  furnish 
much  evidence  that  the  effects  of  use  and  disuse  are  seldom,  if 
at  all,  inheritable  and  hence  have  no  permanency  such  as  the 
characters  of  species  have.  If  the  effects  of  use  and  disuse  are 
not  transmitted,  the  hypothesis  that  the  effects  of  use  and  dis- 
use may  accumulate  from  generation  to  generation  also  lacks 
support.  Also,  in  case  of  plants,  more  recent  investigations  show 
that  modifications  that  are  direct  responses  to  the  environment 
are  not  generally,  if  at  all,  inheritable.  In  the  second  place  his 
explanation  does  not  account  for  the  desire  of  the  animal  to 
change  its  habits,  but  simply  assumes  that  animals  change  in 
their  desires,  and  that  such  changes  are  also  transmitted. 

Darwin's  Explanation.  —  Although  Charles  Darwin  (Fig.  4^4) 
was  neither  the   first   to   believe  in  evolution   nor  the   first  to 


VARIATION 


517 


attempt  to  explain  it,  his  arj^uiuents  for  evolution  were  much 
more  convincing  than  those  of  his  predecessors,  and  his  explana- 
tion was  so  well  based  upon  facts  and  so  well  organized  that  it 
was  widely  accepted.  His  book,  the  Origin  of  Species,  published 
in  1859,  in  which  he  set  forth  his  ;irgument  for  evolution  and  its 
explanation,     was    based 

upon  20  years  of  careful      f  ~  ' 

observat'on,  experiments, 
and  thought,  and  is  one 
of  the  greatest  books  ever 
puljlished.  To  this  book 
is  largely  due  the  accept- 
ance of  the  doctrine  of 
evolution  as  based  upon 
natural  selection,  and 
modern  biology  is  said  to 
date  from  this  book.  The 
two  fundamental  concep- 
tions   of   this    book    are: 

(1)  that  the  process  of 
creation  is  evolution;  and 

(2)  that  the  process  of 
evolution  is  based  upon 
natural  selection. 

Natural  Selection.  — 
According  to  Darwin's 
theory  of  natural  selec- 
tion, the  individuals  of  the  plant  and  animal  world  are  in- 
volved in  continuous  competition  in  nature,  and  only  those 
best  adapted  to  their  surroundings  survive.  Thus  through 
the  destruction  of  those  individuals  not  able  to  survive  in  the 
struggle  for  existence,  a  selective  process,  which  permits  only 
certain  individuals  out  of  a  large  number  to  live  and  propa- 
gate, is  thereby  estabHshed.  Darwin's  theory  of  natural  selec- 
tion involves  five  fundamental  conceptions,  —  variation,  inheri- 
tance, fitness  for  enviromnent,  struggle  for  existence,  and  survival 
of  the  fittest. 

Variation.  —  Variation  is  the  fundamental  fact  in  Darwin's 
theory  of  natural  selection.  By  variations  is  meant  the  devia- 
tions of  organisms  from  a  type  chosen  as  a  standard  of  compari- 


FiG.  464.  —  Charles  Darwin,  the  noted 
scientist,  to  whose  work  the  estabUshment 
of  the  theory  of  evolution  by  natural  selec- 
tion is  chiefly  due. 


518 


EVOLUTION 


son.  Thus  if  one  should  select  a  certain  height,  a  certain  num- 
ber of  flowers  per  plant,  etc.,  as  typical  of  a  given  species  and  as 
standards  of  comparison,  deviations  of  individuals  from  these 
standards    are    variations.     Darwin    was    a    careful    student    of 


Fig.  465.  —  Heads  of  Timothy  selected  from  a  field  of  Timothy  to 
show  variation  in  form  and  size  of  heads.     After  Clark. 


K^-i. 


Fk;. 


lation  in  Icn-tli  of  cars  select. m1  from  a  lield  of  Black 
Me.xican  Sweet  Corn.     .\ft(>r  East. 


variations  and  observed  that  variations  in  structure  and  func- 
tion are  so  common  that  no  two  individual  organisms  are  ex- 
actly alike.  In  a  field  of  grain  or  in  a  group  of  any  of  the  culti- 
vated or  wild  plants  no  two  individuals  can  be  found  that  are 
exactly  alike  (Figs.  1^65  and  1^66).     They  differ  in  height,  shape, 


INHERn-ANCE 


519 


number  of  leaves  or  flowers,  time  of  maturing,  character  of  root 
system,  etc.  Tlic  mdividuals  of  a  group  of  organisms  not  only 
differ  from  each  other,  but  they  also  differ  just  as  strikingly 
from  their  parents.  Even  the  organs  of  the  same  plant,  such 
as  leaves,  vary  widely  (Fig.  467).  Many  of  the  variations  are 
conspicuous,  while  others  are  discovered  only  through  close 
inspection.     Some  variations  better  adjust  the  individual  to  its 


Fig.  467.  —  A  number  of  Mulberry  leaves  selected  from  the  same  tree 
to  show  variation  in  form. 


surroundings,  others  are  detrimental,  and  some  are  neither  use- 
ful nor  detrimental.  Darwin's  idea  was  that  variations,  which 
are  innumerable  and  various  in  kind,  afford  ample  material 
upon  which  natural  selection  can  work.  Why  plants  and  ani- 
mals vary,  Darwin  did  not  attempt  to  explain. 

Inheritance.  —  Although  it  is  common  observation  that  off- 
spring and  parents  differ,  it  is  also  common  observation  that 
there  are  always  some  fundamental  resemblances.  Thus  the 
offspring  of  a  given  variety  of  Corn  are  the  same  in  variety  as 


520  EVOLUTION 

the  parent  and  not  of  another  kind,  although  there  may  be  much 
difference  in  size  of  plants,  length  of  ears,  size  and  shape  of  ker- 
nels, etc.  In  spite  of  variations  there  is  inheritance  which  is 
an  imparting  of  something  by  the  parents  to  the  offspring, 
which  as  a  result  develop  some  of  the  parental  features.  Dar- 
win also  thought  that  not  only  the  resemblances  to  parents,  but 
also  whatever  variations  the  offspring  may  have  are  inheritable. 
Assuming  that  variations  are  transmitted  to  succeeding  gen- 
erations in  which  they  may  become  more  pronounced,  he  could 
explain  the  origin  of  new  characters  and  new  species.  If  a 
variation  is  such  as  to  better  adapt  individuals  to  their  en- 
vironment, those  individuals  in  which  the  variation  is  most 
pronounced  have  a  better  opportunity  to  survive  and  propagate 
in  greater  numbers  than  individuals  in  which  the  variation  is 
less  pronounced.  Thus  through  many  successive  generations  in 
which  the  individuals  with  the  variation  most  pronounced  are 
more  favored  than  other  individuals,  the  variation  is  intensified 
by  natural  selection  and  eventually  becomes  a  character  of  a  new 
species.  Darwin  drew  this  conclusion  from  some  of  his  experi- 
ments as  well  as  from  general  observations  of  plants  and  animals 
under  domestication.  He  observed  that  through  selection  both 
plants  and  animals  changed  much  under  domestication.  Starting 
with  wild  forms  of  plants,  he  was  able  by  selecting  the  variants 
from  the  progeny  for  a  number  of  generations  to  obtain  indi- 
viduals differing  strikingly  from  the  original  wild  forms.  He 
found  that  continuous  selection  gradually  built  up  the  selected 
character  until  the  desired  result  was  obtained.  He  concluded 
that  in  nature  such  a  selective  process  was  brought  about  by 
the  competition  among  individuals  for  existence;  but,  while 
any  character  might  be  built  up  by  artificial  selection,  only  those 
which  enable  the  individual  to  withstand  competition  are  built 
up  in  nature. 

Fitness  for  Environment.  —  It  is  common  observation  that 
different  kinds  of  plants  and  animals  require  different  condi- 
tions under  which  to  live.  As  pointed  out  in  the  discussion  of 
Ecology,  some  plants  and  animals  live  submerged  in  the  water, 
but  most  plants  and  animals  can  live  only  on  land.  The  Cacti 
are  so  constructed  that  they  can  endure  the  drought  of  the 
desert,  but  Corn,  Wheat,  and  most  plants  require  a  medium 
amount  of  moisture.     Again  some  plants  which  are  well  pro- 


STRUGGLE   FOR  EXISTENCE  521 

tected  by  hairs,  like  the  Mullein,  can  thrive  in  the  open  on  dry 
hillsides,  while  there  are  other  plants  that  can  grow  only  in  the 
shade  or  moist  ravines.  The  polar  bear  is  adjusted  to  live 
where  the  climate  is  cold,  while  the  elephant  is  adjusted  to  a 
tropical  climate.  Colored  people  can  live  better  than  white 
people  under  the  tropical  sun  because  of  the  black  pigment  in 
their  skin.  There  are  countless  ways  in  which  plants  and  ani- 
mals show  adjustments  to  particular  kinds  of  environment,  and 
according  to  Darwin  these  adjustments  are  largely  the  results 
of  natural  selection. 

Struggle  for  Existence.  —  That  there  is  an  immense  struggle 
for  existence  in  which  vast  numbers  of  both  plants  and  animals 
perish  is  easily  demonstrated.  The  number  of  seeds  produced 
by  a  plant  and  the  number  of  offspring  that  mature  are  com- 
monly very  different.  For  example,  one  plant  of  the  Russian 
Thistle,  one  of  the  common  weeds,  produces  from  20,000  to 
200,000  seeds.  Taking  25,000  seeds  to  a  plant  as  a  moderate 
estimate,  the  offspring  of  one  plant  would  number  15,625,000,- 
000,000  in  the  third  generation,  if  all  the  seeds  grew.  Allowing 
one  square  foot  per  plant,  the  plants  of  the  third  generation 
would  cover  more  than  500,000  square  miles.  At  this  rate  of 
multiplication,  there  would  be  no  room  in  the  United  States 
for  anything  else  in  a  few  years.  But  the  number  of  plants  that 
develop  is  exceedingly  small  in  comparison  with  the  number  of 
seeds  produced.  Many  of  the  seeds  are  destroyed  before  they 
germinate,  but  many  plants  start  that  do  not  complete  their 
development.  Many  are  killed  by  insects,  and  many  are 
crowded  out  by  more  vigorous  individuals.  On  an  area  three 
feet  long  and  two  feet  wide  which  had  been  dug  and  cleared  so 
that  the  seedlings  were  not  choked  by  other  plants,  Darwin 
found  that  out  of  a  total  of  357  seedHngs  no  less  than  295  were 
destroyed  by  slugs  and  insects.  He  also  measured  off  a  small 
area  of  turf  which  had  long  been  mown  and  allowed  the  plants 
to  grow.  Out  of  the  20  species  of  plants  growing  on  this  small 
plot  of  turf  nine  species,  some  of  which  were  fully  grown,  were 
crowded  out  by  the  more  vigorous  species  and  perished.  The 
same  process  of  ehmination  is  going  on  among  animals.  Even 
in  case  of  the  elephant,  which  is  a  very  slow  breeder,  Darwin 
showed  that  the  progeny  of  a  single  pair  would  number 
19,000,000  in  less  than  800  years,  if  all  survived.     But  since  the 


522  EVOLUTION 

number  of  elephants  in  the  world  remains  practically  the  same, 
many  nmst  perish  for  every  one  that  survives. 

Survival  of  the  Fittest.  —  If  only  a  few  of  the  vast  number  of 
living  things  which  come  into  the  world  survive,  then  the  ques- 
tion as  to  what  individuals  survive  arises.  Darwin's  answer 
is  that  the  survivors  are  those  individuals  having  those  varia- 
tions which  adjust  them  most  perfectly  to  their  surroundings. 
Thus  a  vigorous  growing  plant  will  soon  get  out  of  the  shade  of 
its  neighbors  and  will  also  get  a  larger  proportion  of  the  water 
and  mineral  substances  of  the  soil.  It  will  therefore  succeed  in 
crowding  out  the  less  vigorous  plants  around  it.  Also  the 
plant  that  succeeds  in  developing  the  best  protective  structures 
against  drought,  heat,  bright  sunshine,  or  animals  will  have  an 
advantage  over  the  less  fortunate  ones,  and  will  therefore  sur- 
vive while  the  others  perish.  The  better  adaptation  may  be 
in  the  production  of  more  seeds,  in  better  methods  of  dissemi- 
nating the  seeds,  or  in  numerous  other  ways.  Among  animals 
the  strongest,  fleetest,  most  cunning,  or  best  equipped  for 
fighting  are  the  fittest  and  survive  to  the  expense  of  those  not 
so  well  equipped  for  the  struggle.  To  this  process  of  natural 
selection  Herbert  Spencer  applied  the  phrase  "  survival  of  the 
fittest."  It  is  obvious,  however,  that  this  process  in  nature  is 
more  a  rejection  of  the  unfit  than  a  selection  of  the  fit,  and  it 
has  been  suggested  that  "  natural  rejection  "  would  be  a  more 
appropriate  phrase  for  the  process  than  "  natural  selection." 
The  rejection  of  the  unfit  makes  room  for  the  fit  to  live  and 
perpetuate  their  variations  through  offspring. 

Although  the  variations  which  are  useful  may  be  slight  at 
first,  through  successive  generations  of  selection  Darwin  as- 
sumed that  they  become  more  marked  and  finally  new  char- 
acters are  established  and  thus  new  species  formed.  Of  course 
there  are  many  variations  which  are  neither  of  advantage  nor  of 
disadvantage  to  the  individual.  According  to  Darwin  only 
those  variations  that  are  of  advantage  or  disadvantage  to  the 
individual  are  affected  by  natural  selection. 

Objections  to  Darwinism.  —  Darwin's  theory  aroused  bitter 
antagonism  among  theologians  because  they  thought  it  ehmi- 
nated  God  from  the  plan  of  Creation.  Darwin  was  even  accused 
of  teaching  that  man  descended  from  monkeys.  But  Dar- 
win's theory  neither  eliminates  God  nor  does  it  teach  that  man 


OBJECTIONS  TO   DARWINISM  523 

descended  from  monkeys,  and  theologians  gradually  came  to 
see  that  a  plan  in  which  the  multitudinous  forms  are  evolved 
is  just  as  noble  a  conception  of  God  as  the  "  Doctrine  of  Special 
Creation." 

The  scientists  offered  a  number  of  objections,  some  of  which 
later  investigations  have  answered.  They  said  that,  if  evolu- 
tion by  natural  selection  is  now  in  progress,  one  should  be 
able  to  see  one  species  forming  from  another,  but  such  has  never 
been  observed.  The  absence  of  forms  connecting  two  related 
species,  and  the  presence  of  many  apparently  useless  characters 
among  both  plants  and  animals  they  said  were  not  accounted 
for.  Again,  according  to  geologists  and  astronomers,  the  world 
has  not  been  in  existence  long  enough  for  the  present  forms  to 
be  evolved  by  natural  selection.  The  discovery  of  mutations 
enables  us  to  answer  the  above  objections  as  will  be  noted 
later. 

A  number  of  questions  in  regard  to  his  theory  remain  to  be 
answered  satisfactorily  and  are  receiving  much  attention  at 
the  present  time. 

First,  as  was  stated  under  Lamarck's  explanation  of  evolu- 
tion, there  is  much  evidence  that  acquired  characters,  that  is, 
characters  which  an  individual  does  not  get  from  its  parents 
but  takes  on  during  its  lifetime,  are  seldom  if  at  all  inherited. 
This  strikes  at  Darwin's  assumption  that  useful  variations  are 
finally  estabhshed  as  characters  through  inheritance  and  selec- 
tion. 

Second,  it  is  claimed  that  by  the  selection  of  slight  variations 
new  species  cannot  be  formed.  The  individuals  of  a  species 
can  be  changed,  but  they  can  never  be  changed  so  much  as  to 
form  a  new  species.  In  support  of  this  objection,  it  is  claimed 
that  through  centuries  of  artificial  selection,  plant  and  animal 
breeders  have  not  been  able  to  produce  new  species. 

Third,  although  the  theory  assumes  that  variations  are 
selected  because  of  their  advantage  to  the  individual,  it  also 
assumes  that  useful  variations  may  be  built  up  through  the 
selection  of  variations  which  are  so  slight  at  first  that  they 
give  the  individuals  having  them  no  advantage  over  individuals 
lacking  them.  Also  the  causes  of  variations  Darwin  left  to  his 
successors  to  explain,  and  variations  are  still  a  subject  of  much 
investigation  and  discussion. 


524 


EVOLUTION 


Experimental  Evolution 

The  early  students  of  science  studied  plants  and  animals 
simply  by  observing  them  in  the  field.  They  made  no  effort  to 
control  the  conditions  under  which  the  plants  or  animals  were 
growing.  But  one  can  see  that,  in  order  to  draw  definite  con- 
clusions in  regard  to  the  inheritable  factors  in  plants,  the  ances- 
try of  the  plants  must  be  known,  their  pollination  controlled  so 
as  to  know  definitely  the  parents  of  the  progeny,  and  the  various 
factors  that  affect  the  growth  of  the  plants  must  be  taken  into 

account.  Likewise,  in  the  study 
of  animals  in  reference  to  prob- 
lems of  evolution  it  is  essen- 
tial to  control  their  breeding 
and  often  the  conditions  to 
which  they  are  exposed.  The 
early  scientists  had  poorly 
equipped  laboratories  or  none  at 
all,  and  science  then  was  chiefly 
a  description  of  nature  and  was 
called  "Natural  History." 
Darwin  and  a  few  of  his  con- 
temporaries put  considerable 
emphasis  on  the  experimental 
method,  but  since  Darwin's  time 
the  experimental  method  has 
been  especiall}^  emphasized  with 
the  result  that  rapid  strides 
have  been  made  in  interpreting 
and  explaining  facts. 
Hugo  De  Vries.  —  Hugo  De  Vries  {Fig.  468),  director  of  the 
Botanic  Garden  in  Amsterdam,  Holland,  was  among  the  first 
to  apply  the  experimental  method  to  the  study  of  evolution. 
Starting  with  seed  selected  from  plants  which  he  thought  were 
pure,  that  is,  not  mixed  with  another  variety,  he  grew  a  large 
number  of  generations,  which,  by  carefully  preventing  cross- 
polhnation,  were  kept  pure  to  the  parent  type.  In  order  to 
make  accurate  comparisons  and  thereby  detect  variations  from 
the  parents,  such  as  the  dropping  of  parental  characters  or  the 
taking  on  of  new  ones,  he  not  only  kept  careful  records  but  also 


Fig.  468.  —  Hugo  De  Vries,  whose 
mutation  theory  is  one  of  the  most 
important  contributions  to  the  study 
.of  evokition  since  Darwin's  time.  He 
has  also  made  valuable  contributions 
to  our  knowledge  of  osmosis. 


NATURE  OF  CONTINUOUS  VARIATION  525 

preserved  specimens  of  each  generation.  At  the  same  time  he 
also  carefully  observed  plants  in  the  field,  and,  when  one  was 
found  that  showed  extraordinary  features,  the  experimental 
method  was  applied  to  it.  He  was  especially  interested  in  vari- 
ations, and  his  most  notable  contribution  is  on  this  subject. 
He  demonstrated  by  his  painstaking  work  that  there  are  two 
kinds  of  variations  —  continuous  or  fluctuating  and  discontinu- 
ous or  saltative  variations.  Discontinuous  variations  are  also 
called  mutations  and  are  those  extreme  variations  which  sud- 
denly arise  and  remain  fixed,  that  is,  they  are  transmitted  to 
succeeding  generations. 

Nature  of  Continuous  Variation.  —  Continuous  variations  are 
the  most  common  kind  of  variations.  They  are  simply  the  fluc- 
tuations that  individuals  show  in  size,  shape,  color,  and  other 
characters.  Thus  red  flowers  vary  in  degree  of  redness,  leaves 
vary  in  shape  and  size,  seeds  vary  in  number  per  pod  as  well 
as  in  shape  and  size,  plants  differ  in  height,  shape,  method  of 
branching,  and  so  on.  Continuous  variations  are  chiefly  due 
to  differences  in  the  environment,  such  as  differences  in  sun- 
hght,  food  and  water  supply,  temperature,  and  influences  ex- 
erted by  one  organism  upon  another.  According  to  the  work 
of  De  Vries  and  other  investigators,  they  are  not  inheritable 
and,  therefore,  are  constantly  changing  with  the  conditions  that 
cause  them.  They  fluctuate  around  a  mean  or  average  which 
remains  practically  constant,  and,  above  or  below  this  average, 
the  individuals  varying  gradually  grow  less  in  number  as  vari- 
ability departs  more  and  more  from  the  average  until  a  limit 
in  each  direction  is  reached.  Continuous  variations  follow  the 
law  of  Quetelet,  the  Belgian  anthropologist,  who  found  that 
variability  follows  the  law  of  probabihty.  Small  divergences 
from  the  average  are  numerous,  while  larger  ones  are  less  numer- 
ous, and  the  larger  the  less  numerous  they  are.  If,  for  example, 
a  bushel  or  any  quantity  of  ears  of  Corn  are  separated  into 
piles  according  to  length,  there  will  be  one  length  which  will 
include  the  greatest  number  and,  above  or  below  this  length, 
which  is  known  as  the  average,  the  piles  will  decrease  in  size 
as  the  length  of  ears  in  each  pile  are  greater  or  less  than  the 
average.  This  is  well  illustrated  in  Figure  469  in  which  82  ears 
of  Corn  with  extreme  lengths  4.5  and  9  inches  are  arranged  in 
10  piles  according  to  size.     The  same  fact  is  illustrated  in  Figure 


52t3 


EVOLUTION 


470,  in  which  case  beans  are  assorted  according  to  size.  Of  course 
where  size  is  the  variation  considered,  the  space  occupied  by 
each  class  of  individuals  will  vary  more  than  the  number  of 
individuals  in  each  class,  for  the  smaller  individuals  occupy  less 


ri^ 


Fig.  469.  —  Quetelet's  law  of  continuous  or  fluctuating  variability,  demon- 
strated by  arranging  82  ears  of  Corn  in  piles  according  to  size.  The  ears  were 
taken  from  unselected  material  from  a  field  of  Corn.  The  length  and  number 
of  ears  in  each  pile  are  given  in  figures  below.  Notice  that  the  piles  decrease 
each  way  from  the  pile  (highest  pile)  containing  the  ears  of  average  length, 
decreasing  accordingly  as  the  length  of  ears  in  each  pile  is  greater  or  less  than 
the  average  length.     After  Blakeslee. 


Fig.  470.  —  A  demonstration  of  Quetelet's  law  of  continuous  variation  in 
the  size  of  the  seeds  of  a  Common  Bean.  The  seeds  are  grouped  with  refer- 
ence to  length.  The  longest  column  contains  those  of  average  length  and 
the  columns  to  the  right  or  left  of  it  are  shorter  accordingly  as  the  length  of 
Beans  in  each  column  are  greater  or  less  than  the  average  length.  Redrawn 
from  De  Vries. 


space  than  the  larger  ones.  All  kinds  of  continuous  variations, 
such  as  number  of  flowers  per  plant,  number  of  seeds  per  pod, 
weight  of  seeds,  height  of  plant,  size  and  shape  of  leaves,  etc., 
distribute  themselves  around  an  average  in  the  same  general  way 
as  illustrated  by  size  in  case  of  Beans  and  ears  of  Corn. 


DISCONTINUOUS  VARIATIONS  OR  MUTATIONS  527 

When  a  number  of  generations  of  offspring  are  considered, 
the  continuous  variations  in  each  generation  tend  to  fluctuate 
around  an  average  that  is  common  for  all  the  generations. 
Consequently  the  selection  of  continuous  variations  seldom  im- 
proves the  average,  and,  if  any  improvement  is  obtained  in  this 
way,  it  is  soon  lost  when  selection  is  discontinued.  The  average 
yield  of  sugar  in  Sugar  Beets  has  been  improved  and  is  main- 
tained by  selecting  for  seed  the  plants  having  the  highest  sugar 
content,  but  the  improvement  is  lost  when  selection  is  discon- 
tinued. Johannsen  has  demonstrated  that,  if  one  starts  with  a 
pure  Hne,  that  is,  with  the  offspring  of  a  single  individual  pro- 
duced by  self-fertilization,  and  keeps  the  generations  pure  by 
preventing  cross-pollination,  the  average  of  a  fluctuating  continu- 
ous variation  cannot  be  increased.  He  clearly  demonstrated  this 
with  Beans,  which  self-pollinate  and  hence  remain  pure.  He  at- 
tempted to  increase  the  average  size  of  the  seeds  by  selecting 
the  largest  seeds  of  each  generation  of  a  certain  variety  of  Beans 
for  planting.  He  continued  this  for  a  number  of  generations, 
but  obtained  no  increase  in  the  average  size  of  the  seeds.  Simi- 
lar results  have  been  obtained  by  other  investigators  in  attempt- 
ing to  intensify  certain  desirable  variations  in  Wheat,  Oats,  and 
in  pure  hues  of  other  plants.  For  example,  an  effort  to  increase 
the  yield  in  a  strain  of  Oats  by  selecting  each  year  the  best 
yielding  plants  for  seed  gave  practically  no  increase  in  yield 
after  a  number  of  years  of  selection. 

Discontinuous  Variations  or  Mutations.  —  That  there  are 
two  kinds  of  variations  was  observed  by  Darwin  and  others 
before  and  after  him,  but  De  Vries  was  the  first  to  show  the 
importance  of  discontinuous  variations  in  evolution.  He  formu- 
lated the  theory  that  discontinuous  variations  and  not  con- 
tinuous variations  furnish  the  material  for  natural  selection. 
Discontinuous  variations  differ  from  continuous  variations  in 
that  they  arise  suddenly,  are  usually  of  marked  character,  and 
breed  true,  that  is,  they  are  passed  on  to  the  offspring.  Dis- 
continuous variations  De  Vries  called  mutations.  A  plant  that 
gives  rise  to  mutations  is  said  to  mutate,  and  a  plant  arising  by 
mutation  is  called  a  mutant.  De  Vries's  theory  of  evolution  is 
known  as  the  mutation  theory,  and  its  fundamental  conception 
is  that  species  are  formed  by  the  selection  of  mutations  and  not 
by  the  selection  of  continuous  variations. 


528 


EVOLUTION 


We  now  believe  that  our  cultivated  plants  afford  many  ex- 
amples of  mutations.  Strawberries  without  runners  have  sud- 
denly arisen  among  plants  with  runners  and  have  bred  true  from 
seed.  The  Beseler  Oats,  a  beardless  variety,  originated  from  a  few 
plants  found  in  a  field  of  bearded  Oats.  Also  a  number  of  choice 
new  varieties  of  Wheat  started  from  one  or  a  few  plants  which 


Fig.  471.  —  The  Wild  Cabbage  and  some  of  the  forms  that  are  supposed  to 
be  mutants  of  the  Wild  Cabbage.  A,  Wild  Cabbage;  B,  Kohlrabi;  C,  Cauli- 
flower; D,  Cabbage;  E,  Welsh  or  Savoy  Cabbage;  F,  Brussels  Sprouts. 
After  SmaUan. 


suddenly  appeared  differing  in  characters  from  the  other  plants 
of  the  field.  The  Cauhflower  and  Kohlrabi  (Fig.  471)  were 
raised  from  isolated  monstrosities  of  the  wild  Cabbage  (Brassica 
oleracea).  Green  Roses,  green  Dahlias,  seedless  Oranges,  seed- 
less Bananas,  and  varieties  of  the  Boston  Fern  with  finely  divided 
leaves  are  other  examples  of  mutations.  Sometimes  a  bud  may 
mutate,  giving  rise  to  a  bud  sport.  In  this  way  the  Nectarine, 
which  has  a  fruit   resembling  that  of   the   Peach   but  lacking 


MUTATION   IN  THE  EVENING   PRIMROSE 


529 


the  fuzz  of  a  poach,  sometimes  arises  as  a  ])rauch  of  the  Peach 
tree. 

Mutation  in  the  Evening  Primrose.  —  The  Evening  Primrose 
is  especially  noted  because  it  has  furnished  nmch  of  the  material 
upon  which  the  nmtation  theory  is  founded.  One  of  the  diffi- 
culties in  finding  mutations  is  that  any  given  species  does  not 
mutate  all  of  the  time  but  only  at  occasional  periods.  In 
1886  De  Vries  began  to  search  for  sjoecies  that  were  in  the 
mutating  period.  The 
American  Evening  Prim- 
rose (Oenothera  Lamarck- 
iana)  {Fig.  Jf72),  also 
known  as  La m  a r  c  k  '  s 
Evening  Primrose,  proved 
to  be  the  species  for  which 
he  was  searching.  He 
found  a  large  number  of 
plants  of  this  species 
growing  in  an  abandoned 
potato  field  at  Hilversum, 
near  Amsterdam.  Among 
them  he  found  some  un- 
known and  very  distinct 
forms  which  apparently 
had  come  from  seeds  of 
the  normal  American 
Evening  Primrose.  Seeds 
were  secured  from  the 
normal  plants,  and  cul- 
tures were  begun  in  the 
Botanical  Garden  at  the 
University  of  Amsterdam.  From  the  first  soAving  he  obtained 
•another  new  form.  Through  a  series  of  pedigree  cultures  in- 
volving a  number  of  generations,  quite  a  number  of  distinct 
forms  were  obtained.  Some  of  these  distinctly  new  forms  ap- 
peared repeatedly  in  the  cultures,  while  others  appeared  only 
once,  but  they  all  bred  true,  thus  producing  offspring  like  them- 
selves. These  new  forms  did  not  arise  gradually  but  appeared 
suddenly  and  were  so  distinct  fi'om  the  American  Evening 
Primrose,  their  parent,  as  to  hv  calked  new  species. 


Fig.  472.  — Lamarck's  Evening  Primrose 
{Oenothera  Lamarckiana) ,  a  mutating 
species.     After  De  Vries. 


530 


EVOLUTION 


The  mutations  involved  various  kinds  of  characters.  One  of 
the  new  forms,  Oenothera  brevistylis,  had,  among  other  distinc- 
tive characters,  a  much  shorter  style  than  the  Oenothera  La- 
marckiana.  Another  new  form,  Oenothera  laevifolia,  had  smooth 
leaves  and  much  prettier  foliage  than  the  parent  type,  and  its 
petals  were  not  notched.  One  of  the  finest  and  rarest  of  the 
mutants  was  the  Oenothera  gigas  {Fig.  4'^3),  which  was  stronger, 
bigger,  and  more  heavily  built  than  the  parent.     Others  of  the 

new  forms  difTered  from 
the  parent  in  other  char- 
acters. 

As  to  what  a  new  form 
is,  can  be  determined  only 
by  breeding  it  and  its 
parents  in  pedigree  cul- 
tures. A  new  form  may 
result  from  crossing  be- 
tween two  parents  with 
different  characters  and, 
therefore,  be  a  hybrid. 
The  new  form  may  be 
due  to  the  fact  that  the 
parent  is  a  hybrid  or  de- 
scendant of  a  hybrid  and 
consequently  does  not 
breed  true,  thus  producing 
offspring  different  from 
the  parent  type.  If  the 
parents  are  pure,  and  the 
new  form  is  not  a  hybrid, 
it  must  be  a  mutant  or  a 
fluctuating  variant.  If  it  breeds  true,  it  is  a  mutant;  other- 
wise, it  is  a  fluctuating  variant. 

By  growing  the  parents  of  the  new  forms  in  pedigree  cultures 
for  a  number  of  generations,  he  found  that  they  bred  true, 
except  for  the  new  forms  which  occasionally  appeared,  and 
concluded  that  the  parents  were  pure.  Since  the  parents  of 
the  new  forms  were  carefully  pollinated  artificially,  so  as  to 
prevent  crossing  between  parents  differing  in  characters,  the  new 
forms  were  not  the  result  of  hybridizing.     They  were  either 


Fig.  473.  — The  Giant  Evening  Prim- 
rose (Oenothera  gigas),  one  of  the  mutants 
from  Lamarck's  Evening  Primrose.  After 
De  Vries. 


CAUSES  OF  VARIATION  531 

mutants  or  fluctuating  variants  and,  when  the  test  of  breeding 
true  was  applied,  they  proved  to  be  mutants.  Thus  De  Vries 
had  obtained  mutations  under  experimental  conditions  and  was 
ready  to  announce  the  mutation  th(H)ry. 

The  Mutation  Theory  and  Darwinism.  —  The  nmtation  the- 
ory does  not  disturb  the  theory  of  evolution  by  natural  selection, 
but  holds  a  different  view  as  to  the  material  upon  which  natural 
selection  works.  According  to  Darwinism,  most  all  kinds  of 
variations  are  inheritable,  and  even  though  slight  at  first,  they  can 
become  intensified  through  generations  of  selection  and  finally 
become  distinct  characters  of  new  species.  According  to  the 
mutation  theory,  only  mutations  are  inheritable,  and  they  are 
not  built  up  through  generations  of  selection,  but  arise  suddenly, 
in  full  force,  and  breed  true  thereafter.  Thus  according  to  the 
mutation  theory,  new  species  arise  at  one  bound,  and  all  that  nat- 
ural selection  has  to  do  is  to  determine  whether  they  survive  or 
perish.  If  the  mutation  is  such  that  the  new  species  is  well 
adapted  to  its  surroundings,  then,  according  to  the  law  of  the 
survival  of  the  fittest,  it  will  survive;  otherwise,  it  will  likely 
perish. 

The  mutation  theory  explains  a  number  of  the  early  objections 
to  the  theory  of  natural  selection.  It  accounts  for  the  fact  that 
species  have  so  many  characters  which  are  apparently  of  no 
value  in  fitting  the  individual  to  live.  It  is  obvious  that  a 
species  may  have  characters  of  no  importance  as  well  as  useful 
ones,  if  new  characters  arise  at  a  bound  in  full  force,  and  their 
presence  does  not  depend  upon  their  having  met  the  test  of  fitting 
the  species  to  live  through  generations  of  selection.  The  muta- 
tion theory  accounts  for  the  absence  of  intermediate  forms  or 
so-called  connecting  links  between  species.  Evolution  by  mu- 
tations requires  less  time  than  evolution  by  the  natural  selec- 
tion of  fluctuations,  and  in  this  way  the  theory  answers  the 
objection  to  the  lack  of  time. 

Causes  of  Variations.  —  There  are  various  causes  which  have 
to  do  with  bringing  aliout  variations  in  both  plants  and  ani- 
mals, many  of  which  are  not  understood.  Numerous  varia- 
tions are  due  directly  to  differences  in  food  supply,  climatic 
conditions,  and  other  external  factors  to  which  the  individual 
is  exposed.  Thus  a  plant  well  situated  in  reference  to  light,  so 
that  its  leaves  can  make  carbohydrates  abundantly,  is  likely  to 


532  EVOLUTION 

be  larger  than  a  plant  that  is  shaded.  Likewise,  the  amount  of 
soil  moisture  and  mineral  matter  available,  the  amount  of 
transpiration  to  which  the  plant  is  exposed,  the  temperature  of 
the  soil  and  atmosphere,  and  the  velocity  of  winds,  and  the 
amount  of  competition  with  other  plants  are  also  common  en- 
vironmental factors  that  cause  variations.  Variations  which 
are  responses  to  variations  in  the  environment  are  seldom,  if 
at  all,  inheritable  and  hence  are  fluctuating  variations.  Fluctu- 
ating variations  in  individuals  may  be  due  also  to  fluctuating 
variations  in  parents.  Thus,  if  a  parent  plant  is  poorly  nour- 
ished, its  seeds  may  be  poorly  developed  and  produce  offspring 
that  vary  from  the  ordinary  type  in  size  and  vigor. 

Fluctuating  variations  also  arise  as  a  result  of  sex.  Plants 
produced  by  vegetative  propagation  are  often  less  vigorous  than 
those  grown  from  seed.  The  fusion  of  a  sperm  and  an  egg  in 
fertihzation  often  results  in  a  rejuvenating  effect  which  is  mani- 
fested in  a  more  vigorous  offspring.  The  relationship  of  the 
sperm  and  egg  involved  in  fertilization  has  an  important  bearing 
upon  the  variations  in  the  offspring.  For  example,  in  Corn  the 
offspring  resulting  from  self-fertilization  is  not  nearly  so  vigor- 
ous as  offspring  resulting  from  a  fertilization  in  which  the  sperm 
and  egg  come  from  different  parents,  although  the  parents  are 
the  same  in  type.  In  addition  to  fluctuating  variations  and 
mutations,  there  are  those  numerous  differences  among  indi- 
viduals due  to  heredity,  such  as  occur  in  the  offspring  when 
parents  differing  in  variety  or  species  are  crossed.  In  connec- 
tion with  these  differences  due  to  heredity,  some  fluctuating 
variations,  such  as  variations  in  size  and  vigor,  commonly  occur. 

Mutations  are  apparently  caused  by  changes  within  the  in- 
dividual, and,  although  environmental  factors  may  have  much 
to  do  with  bringing  them  about,  they  are  not  direct  responses 
to  variations  in  the  environment.  Usually  they  involve  the  en- 
tire constitution  of  the  individual,  the  gametes  as  well  as  vegeta- 
tive structures,  wliile  fluctuating  variations  usually  involve  only 
vegetative  structures. 

Somatoplasm  and  Germ-plasm.  —  Weismann,  a  German  bi- 
ologist, and  his  followers  hold  the  theory  that  a  plant  or  animal 
consists  of  two  kinds  of  protoplasms,  which  act  more  or  less 
independently  of  each  other.  The  protoplasm  of  which  sperms 
and  eggs  are  formed  they  call  germ-plasm,  while  all  protoplasm 


SOMATOPLASM   AND   GERM-PLASM  533 

tliat  does  not  have  to  do  directly  with  forming  sex  cells  is  called 
somatoplasm.  Thus  the  protoplasm  of  all  vegetative  structures 
of  plants,  such  as  leaves,  roots,  and  stems,  is  somatoplasm. 
Even  the  parts  of  a  flower,  excepting  the  protoplasm  immedi- 
ately involved  in  the  production  of  sex  cells,  is  somatoplasm. 
According  to  Weismannism,  the  characters  of  a  species  are  de- 
termined by  certain  units  or  factors  within  the  germ-plasm 
and  the  germ-plasm  remains  practically  the  same  from  genera- 
tion to  generation  in  respect  to  the  factors  contained,  although 
the  factors  may  change  in  reference  to  each  other.  He  holds 
that  changes  in  the  somatoplasm,  such  as  those  variations  in 
leaves,  roots,  and  stems  that  occur  in  response  to  environmental 
influences,  are  not  imparted  to  the  germ-plasm  and  consequently 
are  not  inheritable.  This  theory  that  the  germ-plasm  remains 
practically  the  same  throughout  generations  is  known  as  the  con- 
tinuity of  the  germ-plasm.  Since  modifications  in  the  somato- 
plasm leave  no  trace  of  themselves  in  the  germ-plasm,  it  is  obvi-. 
ous  that,  according  to  Weismann's  view,  characters  cannot  be 
acquired.  This  means  that  any  trait  which  an  individual  does 
not  inherit  but  acquires  during  its  life  time  in  response  to  envi- 
ronment is  not  transmitted  to  the  offspring.  Thus,  if  a  parent 
acquires  great  skill  as  a  musician,  mathematician,  or  in  other 
lines,  the  children  of  this  parent  inherit  none  of  this  acquired 
ability.  Also,  in  case  of  plants,  the  particular  modifications 
which  individuals  take  on  during  their  life  time  disappear  with 
the  individuals. 

In  accounting  for  the  inheritable  changes  occurring  in  indi- 
viduals and  generations,  Weismann  tells  us  that  the  units  or 
factors  in  the  germ-plasm  are  changing  in  relation  to  each 
other,  and  these  changes  account  for  the  origin  of  new  char- 
acters. Some  units  may  become  stronger  and  others  weaker, 
and  they  may  combine  in  various  ways.  Such  changes  may  be 
induced  by  external  conditions,  such  as  poor  nourishment, 
drought,  competition,  etc.,  but  the  character  resulting  there- 
from may  be  of  any  kind,  and  hence  only  by  chance  is  it  of  such 
a  nature  as  to  adjust  the  individual  to  its  environment.  For 
example,  the  changes  induced  in  the  units  of  germ-plasm  by  an 
environmental  factor,  such  as  drought,  may  result  in  a  change 
in  the  color  of  the  flower,  length  of  style,  arrangement  of  leaves, 
etc.     Thus  the  character  resulting  from  the  change  induced  by 


534  EVOLUTION 

the  environment  may  show  no  adaptive  connection  whatever 
with  the  external  influence  inducing  the  change.  Another  cause 
of  changes  among  the  units  in  the  germ-plasm  Weismann  attrib- 
utes to  interactions  upon  each  other  of  different  units  brought 
together  through  fertiUzation. 

According  to  Weismann's  theory  of  the  constitution  of  organ- 
isms, fluctuating  variations  are  due  to  changes  only  in  the 
somatoplasm,  while  mutations  are  due  to  shifts  among  the 
factors  in  the  germ-plasm.  Mutations  arise  as  a  result  of  changes 
within  the  individual,  while  fluctuating  variations  usually  arise  in 
response  to  influences  external  to  the  individual.  Although  the 
external  influences  causing  fluctuating  variations  may  at  the  same 
time  so  indirectly  influence  the  germ-plasm  as  to  cause  mutations, 
the  fluctuating  variations  themselves  involve  only  the  somato- 
plasm, and  hence  are  not  recorded  in  the  germ-plasm.  For 
example,  a  lack  of  moisture  may  cause  such  fluctuating  variations 
as  a  reduction  in  size  of  leaves,  fruit,  and  in  number  of  flowers 
produced  per  plant,  and  at  the  same  time  induce  such  changes  in 
the  germ-plasm  that  a  mutation  in  color  of  flowers,  length  of 
style,  etc.,  may  appear  in  the  offspring,  but  the  fluctuating  varia- 
tions disappear  with  the  vegetative  structures  involved. 

Weismann's  theories  have  not  been  sufficiently  demonstrated, 
and  there  are  some  objections  to  them.  Although  the  evidence 
seems  to  be  against  the  inheritance  of  acquired  characters,  this 
question  is  not  yet  settled. 


CHAPTER  XXIII 

HEREDITY 

General  Features  of  Heredity 

Nature  of  Heredity.  —  The  constancy  of  species  of  plants 
and  animals  through  successive  generations  depends  upon  the 
fact  that  the  individuals  of  each  generation  are  fundamentally 
like  their  parents.  Thus  the  offspring  of  Sweet  Corn  have  the 
characters  of  Sweet  Corn,  and  the  offspring  of  Flint  Corn  have 
the  characters  of  Fhnt  Corn.  Even  when  parents  differing  in 
one  or  more  fundamental  characters  are  crossed,  the  characters 
of  both  parents  will  appear  somewhere  in  future  generations. 
The  transmission  from  parent  to  offspring  of  similarities  in 
structure  and  function  is  heredity.  But  heredity  means  more 
than  the  transmission  of  similarities.  In  the  study  of  varia- 
tions it  was  learned  that  no  two  individuals  are  alike,  and  hence 
offspring,  although  fundamentally  similar  and  like  their  parents, 
always  have  individual  differences.  Thus  every  plant  or  ani- 
mal, besides  resembling  its  parents,  brothers,  and  sisters,  has  its 
own  peculiar  features  which  give  it  individuality.  In  a  field  of 
a  given  variety  of  Corn,  although  the  plants  have  the  characters 
of  the  parent  variety,  they  differ  in  height  of  stalk,  length  and 
shape  of  ear,  depth  of  kernel,  time  of  maturing,  and  in  many 
other  ways.  Most  of  these  differences  are  fluctuating  varia- 
tions, while  some  may  be  due  to  something  inherited.  In  the 
study  of  heredity  not  only  the  resemblances  but  also  the  differ- 
ences must  be  accounted  for.  Heredity  means  the  transmission 
of  fundamental  resemblances  with  differences  in  detail. 

The  Physical  Basis  of  Heredity.  —  It  is  obvious  that  parents 
do  not  actually  transmit  characters  to  the  offspring,  but  trans- 
mit something  that  causes  the  characters  to  appear  in  the 
offspring.  For  example,  a  red  flowered  Sweet  Pea  does  not 
transmit  a  red  color  to  the  flowers  of  its  offspring,  but  transmits 
something  through  the  sperm  and  egg  that  causes  a  red  color 


536  HEREDITY 

to  appear  in  the  flowers  of  its  progeny,  Tlie  exact  nature  of 
tlie  factors  or  substances  which  are  responsible  for  the  appear- 
ance of  characters  is  not  well  understood,  but  it  is  quite  evident 
that  they  are  protoplasmic  substances.  That  they  are  proto- 
plasmic substances  is  probably  more  easily  demonstrated  in 
the  lower  than  in  the  higher  organisms.  In  the  reproduction  of 
simple  one-celled  plants,  like  Pleurococcus,  the  plotoplasm  of 
the  parent  divides,  and  each  half  of  the  parent  becomes  a  new 
individual.  The  parent  thus  disappears  in  the  formation  of 
new  individuals  or  progeny,  which  are  at  first  merely  segments 
of  the  parent.  The  new  individuals,  as  they  develop  to  normal 
size,  develop  in  full  the  features  characteristic  of  the  parent. 
They  separate  soon  after  they  are  formed,  develop  a  lobed  chlo- 
roplast,  enlarge  and  thicken  their  walls  with  cellulose,  and  retain 
a  globular  form.  These  are  the  constant  characteristics  by  which 
we  know  Pleurococcus,  despite  the  fact  that  there  are  numerous 
other  ways  the  plant  might  develop.  It  might,  for  example, 
form  a  filament  like  that  of  Spirogyra,  develop  ribbon-like 
chloroplasts,  and  enclose  itself  in  a  woody  wall.  The  charac- 
ters of  Pleurococcus  are  the  results  of  the  way  the  protoplasm 
works,  for  the  protoplasm  forms  the  chloroplasts,  the  cell  walls, 
and  is  responsible  for  the  separation  and  shape  of  the  cells.  It 
is  obvious  that  the  characters  of  this  simple  plant  are  what 
they  are  and  are  constant  because  the  protoplasm  has  a  dispo- 
sition to  work  only  in  certain  ways  and  retains  this  particular 
disposition  as  it  passes  from  generation  to  generation. 

In  higher  plants  and  animals  the  parent  does  not  divide  and 
each  half  go  to  form  a  new  individual,  but  only  a  small  part  of 
the  parent,  a  sperm  and  an  egg,  are  transmitted  to  the  offspring. 
Hence  the  sperm  and  egg  must  contain  all  of  the  protoplasmic 
constituents  necessary  for  producing  in  the  offspring  the  char- 
acters of  the  parents.  But  the  sperm  consists  almost  entirely 
of  a  nucleus,  and  hence  it  is  believed  that  the  material  upon 
which  the  development  of  characters  depends  is  within  the 
nucleus,  and  occurs  in  connection  with  the  chromatin,  which,  in 
the  form  of  chromosomes,  behaves  in  such  a  regular  way  during 
cell  division  as  to  suggest  a  definite  relation  to  heredity.  These 
pi'otoplasmic  constituents  upon  which  the  development  of  char- 
acters depends,  Weismann  called  determinants,  and  some  other 
biologists  call  them  genes. 


NEED  OF  EXPERIMENTAL  STUDY  537 

Active  and  Latent  Genes.  — ■  Not  all  of  the  genes  inherited 
manifest  themselves,  for  many  lie  dormant  and  consequently 
cause  no  corresponding  characters  to  develop.  Sometimes  they 
lie  dormant  for  a  generation  or  more  and  then  become  active. 
On  this  account  offspring  may  have  ancestral  characters  which 
the  parents  did  not  have.  Certain  genes  for  male  characters 
are  always  latent  in  females.  Thus  a  mother  may  transmit  to 
her  son  genes  for  a  beard  like  her  father's,  although  she  develops 
no  beard  herself.  It  is  obvious  that  one  cannot  accurately  judge 
the  constitution  of  a  plant  or  animal  by  the  characters  present, 
for  judgment  based  upon  external  appearances  takes  no  account 
of  the  latent  genes.  Further  breeding  is  necessary  to  determine 
':hese.  Latent  genes  becoming  active  is,  therefore,  one  of  (:he 
reasons  why  offspring  differ  from  their  parents. 

Importance  of  the  Study  of  Heredity.  —  The  study  of  her- 
edity occupies  an  important  position  among  the  sciences.  The 
study  of  heredity  has  for  its  aim  the  discovery  of  the  laws  of 
heredity,  and  a  knowledge  of  these  laws  is  essential  in  many 
lines  of  work.  The  improvement  of  our  crop  plants  and  domes- 
tic animals  through  breeding  depends  upon  the  laws  of  heredity. 
The  science  of  sociology  and  all  sciences  dealing  with  the  physical, 
moral,  and  spiritual  welfare  of  humanity  are  more  or  less  con- 
cerned with  the  laws  of  heredity.  Even  in  the  realm  of  medi- 
cine the  laws  of  heredity  are  taken  into  account. 


Experimental  Study  of  Heredity 

Need  of  Experimental  Study.  —  In  the  discussion  of  evolution 
it  was  shown  that  the  application  of  the  experimental  method 
to  the  study  of  evolution  has  added  much  to  our  knowledge 
of  this  subject.  We  had  no  definite  knowledge  of  the  kinds  of 
variations  and  their  relation  to  natural  selection  until  they 
were  studied  by  the  experimental  method.  In  the  study  of 
heredity  the  experimental  method  is  as  essential  as  in  the  study 
of  evolution.  Heredity  is  so  apparent  in  both  plants  and  ani- 
mals that  we  are  all  convinced  of  its  reality.  That  children 
resemble  their  parents  in  habits,  disposition,  color  of  eyes  and 
hair,  and  other  features  are  so  noticeable  that  they  are  matters 
of  common  observations.  Casual  observations,  however,  fall 
short  of  giving  us  a  knowledge  of  the  laws  of   heredity.     The 


538  HEREDITY 

discovery  of  the  laws  of  heredity  requires  carefully  planned  and 
systematic  work.  One  must  obtain  a  more  or  less  accurate 
knowledge  of  the  history  of  the  parents,  control  the  breeding 
so  as  to  know  the  exact  parents  of  the  offspring,  carefully  study 
each  individual  of  the  offspring  so  as  to  discover  the  hereditary 
relationship  between  parents  and  offspring  and  between  the 
different  individuals  of  the  offspring.  One  must  also  take  into 
account  the  conditions  which  affect  the  plants  or  animals  of  the 
experiment.  Carefully  recorded  facts  obtained  by  experimental 
study  involving  a  number  of  generations  and  various  kinds  of 
plants  and  animals  afford  a  basis  for  conclusions  concerning  the 
laws  of  heredity.  The  experimental  study  of  heredity  as  just 
described  is  known  as  genetics,  a  subject  in  its  infancy  but  one 
of  th(^  most  popular  and  most  promising  of  the  sciences. 

Biometry.  —  Before  the  experimental  method  of  studying 
heredity  came  into  common  use,  the  statistical  method  was 
employed.  The  investigators  who  study  heredity  by  the  sta- 
tistical method  of  recording  data  are  called  biometricians.  In- 
stead of  dealing  with  the  variations  of  single  individuals,  they 
deal  with  the  average  variations  of  a  mass  of  individuals  or 
populations.  It  is  a  method  of  discovering  how  masses  of 
individuals  behave  through  a  series  of  generations,  and  not  a 
method  of  discovering  how  individuals  behave.  For  example, 
they  determine  whether  or  not  the  average  yield,  average 
height,  or  average  weight  of  a  population  is  remaining  constant 
or  shifting,  and  such  information  is  often  valuable.  By  keeping 
a  record  of  the  average  yield  per  acre  of  different  strains  of 
Wheat  for  a  number  of  years,  the  strain  yielding  best  can  be 
determined.  By  such  records  one  can  also  detemine  whether 
or  not  newly  introduced  strains  or  varieties  hold  up  in  yield. 

The  biometricians  formulated  some  laws  of  heredity.  Francis 
Galton,  who  was  one  of  the  foremost  of  the  biometricians,  formu- 
lated a  law  of  heredity  and  announced  it  in  1897.  This  law 
states  that  to  the  total  heritage  of  the  offspring  the  parents  on 
an  average  contribute  |,  the  grandparents  j,  and  the  great  grand- 
parents I,  and  so  on,  the  total  heritage  being  taken  as  unity. 

The  objection  to  the  method  employed  by  the  biometricians 
is  that  it  does  not  pay  enough  attention  to  the  variations  of 
individuals.  A  mass  of  individuals,  such  as  a  field  of  Wheat 
or  Corn  grown  from  the  purest  market  seed,  is  a  mixture  of  in- 


GREGOR   MENDEL 


539 


dividuals  differing  in  heritage,  and  not  much  can  l)e  determined 
concerning  the  laws  of  lieredity  from  such  a  mixture.  Tlie 
average  means  nothing  unless  the  individuals  measured  or 
counted  are  alike  in  their  heritage,  and  the  only  way  to  be  sure 
that  the  individuals  of  a  mass  or  population  are  homogeneous  in 
constitution  is  to  pedigree  them,  that  is,  grow  them  all  from  a 
common  stock.  The  importance  of  pedigree  cultures  is  well 
shown  in  Mendel's  work. 

Gregor  Mendel.  —  Gregor  Mendel  (1822-1884)  {Fig.  474), 
was  an  Austrian  monk  and  abbott  in  the  monastery  of  Briinn, 
where  he  conducted  his  ex- 
periments in  the  Cloister 
Garden.  He  loved  plants 
and  loved  to  experiment  with 
them.  Although  he  studied 
heredity  only  as  a  pastime, 
his  laws  of  heredity  and  his 
experimental  method  of  in- 
vestigating them  are  two  of 
the  most  important  contribu- 
tions ever  made  to  biological 
science. 

Mendel's  success  was  due  to 
the  clearness  with  which  he 
thought  out  the  problem.  He 
knew  the  works  of  other  in- 
vestigators of  heredity,  and 
attributed  their  failure  to 
reach  definite  conclusions  to 
a  want  of  precise  and  con- 
tinued analysis.  To  obtain 
definite  results  he  saw  that  it  was  necessarj^  to  start  with  pure 
material,  to  consider  each  character  separately,  and  to  keep  the 
different  generations  distinctly  separate.  He  also  realized  that 
the  progeny  of  each  individual  must  be  recorded  separately. 
Such  ideas  were  new  in  Mendel's  time,  but  he  felt  certain  that 
experiments  carried  on  in  this  systematic  way  would  give  regu- 
lar results  and  lead  to  definite  conclusions. 

Mendel  saw  that   most   could   be  accomplished   by  crossing 
plants  of  different  varieties  or  species  and  observing  the  be- 


FiG.  474.  —  Gregor  Mendel,  whose 
theory  of  inheritance  is  the  most  im- 
portant contribution  ever  made  to  our 
knowledge  of  heredity. 


540  HEREDITY 

havior  of  the  hybrid  offspring  in  successive  generations.  His 
plan  was  to  cross  plants  differing  in  one  or  a  few  outstanding 
characters,  such  as  the  color  of  flowers,  height  of  plant,  color  and 
shape  of  seeds,  etc.,  and  determine  the  laws  governing  the  appear- 
ance of  these  characters  in  the  hybrid  offspring. 

Material  Chosen,  —  Mendel  used  great  care  in  the  selection 
of  the  plants  to  be  used  in  the  experiment,  for  he  realized  that 
the  success  of  any  experiment  depends  upon  the  choice  of  the 
most  suitable  material.  After  careful  consideration  he  chose 
the  Edible  or  Garden  Pea  as  the  chief  plant  with  which  to  work. 
The  Garden  Pea  proved  to  be  the  most  suitable  plant  because: 
(1)  it  matures  in  a  short  time;  (2)  varieties  in  cultivation  are 
distinguished  by  striking  characters  recognizable  without  trouble; 
(3)  the  plants  are  self-fertilized,  and  hence  plants  chosen  for 
parents  would  be  pure,  that  is,  they  would  have  no  hybrid 
blood  in  them,  and  the  hybrids  and  their  offspring  would  not  be 
disturbed  by  crossing  in  the  successive  generations. 

Mendel's  Experiments.  —  Mendel  investigated  pairs  of  char- 
acters separately  and  in  relation  to  each  other,  and  extended  his 
investigations  to  include  many  pairs  of  characters,  in  order  to  see 
if  all  appeared  in  the  successive  generations  of  offspring  accord- 
ing to  the  same  law.  His  method  of  procedure  may  be  shown 
by  describing  his  experiments  with  tall  and  dwarf  Peas.  A  tall 
Pea,  having  a  height  of  6-7  feet  was  crossed  with  a  dwarf  Pea, 
having  a  height  |  to  1|  feet.  By  means  of  forceps  or  other  in- 
struments and  before  the  flowers  were  open,  the  anthers  were  re- 
moved from  the  flowers  of  the  plant  selected  as  the  mother  plant, 
and  pollen  from  the  pollen  parent  was  applied  to  the  stigma. 
The  hybrid  seeds  developed  by  the  mother  plant  as  a  result  of  the 
crossing  were  carefully  collected.  These  cross-bred  seeds  were 
planted  and  produced  the  first  hybrid  generation  of  plants,  known 
as  the  Fi  generation  in  our  modern  terminology.  The  height  of 
each  individual  of  this  generation  was  carefully  noted,  and  each 
individual  was  compared  with  the  parents  in  respect  to  tallness 
or  dwarfness.  The  individuals  of  this  generation  were  allowed 
to  self-fertilize,  and  the  seeds  of  each  individual  were  collected 
and  planted  separately.  From  these  seeds  he  grew  the  second 
generation  or  F^  generation  according  to  modern  terminology. 
The  individuals  of  the  progeny  of  each  of  the  Fi  plants  were 
carefully  compared  with  their  parents  and  grandparents  in  re- 


MENDEL'S   DISCOVERIES  541 

spect  to  tallness  and  dwarfness  and  tho  facts  carefully  recorded. 
The  individuals  of  the  F^  generation  were  allowed  to  self-ferti- 
lize, and  from  the  seeds  obtained  the  Fz  generation  was  grown, 
and  the  individuals  in  this  generation  were  studied  in  the  same 
careful  wixy  as  those  of  the  previous  generations.  Throughout 
a  number  of  generations  the  behavior  of  tallness  and  dwarfness 
was  carefully  recorded.  In  this  way  he  studied  many  pairs  of 
characters  such  as:  (1)  shape  of  pod  (whether  simply  inflated 
or  deeply  constricted  between  the  seeds);  (2)  color  of  unripe 
pod  (whether  green  or  yellow);  (3)  distribution  of  flowers  on 
the  stem  (whether  distributed  along  the  axis  of  the  plant  or 
bunched  at  the  top);  (4)  color  of  cotyledons  (whether  yellow 
or  green);  (5)  shape  of  seeds  (whether  round  or  wrinkled); 
and  (6)  color  of  seed  coat  (whether  gray  or  brown,  with  or 
without  violet  spots,  or  white). 

Mendel's  Discoveries.  —  Mendel  found  that  in  most  cases  the 
different  pairs  of  characters  investigated  behaved  in  the  same 
way  and  appeared  in  a  regular  way  in  the  successive  generations. 
Furthermore,  it  made  no  difference  as  to  which  variety  was  used 
as  the  mother  parent.  In  case  of  tallness  and  dwarfness,  all  the 
plants  of  the  first  or  Fi  generation  were  tall.  They  were  all  like 
the  tall  parent.  In  the  second  or  F2  generation  there  were  both 
tall  and  dwarf  plants,  but  there  were  three  times  as  many  tall 
plants  as  dwarf  ones,  the  tails  and  the  dwarfs  thus  occurring 
in  the  ratio  of  3  :  1 .  The  offspring  of  the  dwarfs  were  all  dwarfs 
in  the  third  or  F^  generation  and  in  all' succeeding  generations. 
The  dwarfs,  therefore,  were  pure  for  dwarfness,  that  is,  they 
had  no  factors  or  genes  for  tallness  in  them.  One  out  of  every 
three  tall  plants  also  bred  true  and,  therefore,  proved  to  be 
pure  for  tallness,  but  two  out  of  every  three  tall  ones  gave  three 
times  as  many  tall  ones  as  dwarfs  or  a  ratio  of  3  :  1,  thus  being 
apparently  the  same  in  constitution  as  each  of  the  individuals  of 
the  Fi  generation.  They  evidently  contained  factors  or  genes 
for  both  tallness  and  dwarfness.  The  dwarfs  and  one-third  of 
the  tall  ones  of  the  7^3  progeny  bred  true,  while  two-thirds  of 
the  tall  ones  again  bred  as  in  the  previous  generation,  giving  the 
ratio  3:1,  and  two-thirds  of  the  tall  ones  being  impure.  This 
proved  to  be  a  constant  way  of  behaving  throughout  genera- 
tions. The  character  of  the  individuals  of  the  different  gen- 
erations are  shown  in  Figure  1^.75.     Thus  by  the  further  breed- 


542 


HEREDITY 


Fig.  475.  —  A  diagram  illustrating  Mendel's  discovery  concerning  the  in- 
heritance of  tallness  and  dwarfness  in  the  Garden  Pea.  At  the  top  are  the 
parents  of  the  cross,  th(^  tall  variety  at  the  left  and  the  dwarf  variety  at  the 
right.  In  the  line  immo(Hately  below  is  the  first  (Fi)  generation,  all  plants  of 
which  are  tall  and  thus  like  the  tall  parent,  and  each  of  which  upon  being  self- 
fertilized  produced  a  progeny  {Ft  generation)  consisting  of  tall  and  dwarf 
plants  in  the  ratio  of  3  :  1,  as  shown  in  the  third  line.  As  shown  in  the  lower 
line  (/^.■i  generation),  the  dwarfs  and  one-third  of  tails  of  the  Fi  generation  bred 
true,  that  is,  they  produced  progeny  like  themselves,  while  two-thirds  of  the 
tall  ones  produced  a  progeny  consisting  of  tall  and  dwarf  plants  in  the  ratio 
of  3  :  1. 


SEGREGATION   AND   PURITY  OF   GAMETES  543 

ing  of  the  second  hyl)rid  genemtion,  it  was  found  that  although 
the  tails  and  dwarfs  appeared  in  the  ratio  of  3  :  1,  there  were  in 
reality  three  kinds  of  plants,  pure  tails,  impure  tails,  and  pure 
dwarfs,  occurring  in  the  ratio  1:2:1,  and  that  the  impure  tails 
always  produced  three  kinds  of  plants  in  the  same  ratio  of  1  :  2  : 1. 
Law  of  Dominance.  —  It  is  obvious  that  tallness  dominated 
dwarfness  in  the  hybrid  Peas,  and  this  accounts  for  the  fact  that 
all  of  the  first  generation  were  tall,  although  all  of  them  had  genes 
for  dwarfness  as  well  as  for  tallness  in  them.  It  also  explains  why 
the  impure  tall  ones  in  succeeding  generations  were  tall,  although 
they  had  genes  for  dwarfness  in  them.  In  extending  his  investi- 
gations to  other  pairs  of  characters,  Mendel  found  that  smooth- 


D  X  R  first  parent  generation 

i 
D{R)  first  hybrid  generation 

i 


I 

i 
ID  2D(R)  IR     second  hybrid  generation 

I 


~i     I       r 

D     ID     2D{R)      \R     R      third  hybrid  generation 

Fig.  476.  —  Diagram  illustrating  the  constitution  of  the  individuals  of 
the  first,  second,  and  third  hybrid  generation  with  reference  to  dominant 
(D)  and  recessive  characters  {R). 

ness  of  seeds  dominated  wrinkledness,  yellow  color  of  cotyledons 
dominated  green,  and  so  on.  Such  pairs  of  contrasting  characters 
are  called  allelomorphs,  and  the  one  dominating  is  known  as  the 
dominant  and  the  other  as  the  recessive  character.  Mendel's 
law  of  dominance  may  be  stated  as  follows:  When  pairs  of 
contrasting  characters  are  combined  in  a  cross,  one  character 
behaves  as  a  dominant  while  the  other  behaves  as  a  recessive. 
Representing  the  dominant  character  by  D  and  the  recessive 
by  R,  the  behavior  of  dominant  and  recessive  characters  are  as 
shown  in  the  diagram  in  Figure  ^76. 

Segregation  and  Purity  of  Gametes.  —  Since  the  pure  tall  and 
pure  dwarf  plants  of  the  second  generation  and  succeeding 
generations  showed  no  tendency  to  produce  anything  but  pure 
tall  or  pure  dwarf  plants,  they  evidently  had  no  genes  or  parts 
of  genes  for  the  contrasting  characters.     The  genes  for  con- 


544  HEREDITY 

trasting  characters  must  have  separated  as  units,  and  in  the 
separation  no  straggHng  part  of  a  gene  was  left  associated  with 
the  gene  for  the  contrasting  character.  To  this  complete  sep- 
aration of  genes  and  consequently  contrasting  characters  the 
term  segregation  was  applied. 

The  complete  segregation  ot  characters  also  implies  a  purity 
of  gametes.  The  constitution  of  an  individual  depends  upon 
what  the  sperm  and  egg  introduced  into  the  fertilized  egg  from 
which  the  individual  developed.  Thus,  if  a  plant  is  pure  for 
tallness,  the  sperm  and  egg  involved  in  the  fertilization  resulting 
in  the  production  of  this  plant  could  not  have  contained  genes 
for  dwarfness.  Of  the  two  kinds  of  genes,  they  contained  only 
those  for  tallness.  The  same  is  true  in  case  of  dwarfness  or  any 
other  one  of  a  pair  of  contrasting  characters.  This  really  means 
that  in  a  fertilization  resulting  in  the  production  of  a  plant 
pure  for  a  character  both  the  sperm  and  egg  have  genes  for  the 
same  contrasting  character,  and  that  an  individual  pure  in  re- 
spect to  a  character,  therefore,  is  one  that  has  inherited  from 
both  parents  genes  for  the  same  character.  In  other  words,  a 
plant  pure  for  a  character  is  one  that  receives  a  double  dose  of 
genes  for  this  character.  On  the  other  hand,  plants,  hke  the 
impure  tall  ones,  have  received  genes  for  the  dominant  character 
from  one  parent  and  genes  for  the  recessive  character  from  the 
other  parent,  and  hence  they  have  only  a  single  dose  of  genes 
for  either  of  the  characters.  Such  a  plant  we  now  speak  of  as 
being  heterozygous,  while  plants  having  a  double  dose  of  genes 
and  hence  pure  for  a  character  are  regarded  as  homozygous. 
Since  plants  pure  for  a  character  breed  true,  their  gametes  must 
be  alike  in  respect  to  genes  contained.  The  descendants  of  a 
homozygous  parent  propagated  entirely  by  self-fertihzation  are 
of  course  pure  and  constitute  what  is  known  as  a  pure  line. 

After  tracing  the  behavior  of  single  pairs  of  characters  through 
successive  generations,  he  took  up  the  study  of  two  or  more  pairs 
of  contrasting  characters,  the  aim  being  to  determine  how  pairs 
of  contrasting  characters  behave  in  respect  to  each  other.  For 
example,  he  crossed  Peas  characterized  by  smooth  yellow  seeds 
with  Peas  characterized  by  wrinkled  green  seeds.  In  this 
case  he  was  dealing  with  two  pairs  of  characters,  smooth  and 
wrinkled,  and  yellow  and  green,  with  smooth  and  yellow  as 
dominants.     He  found  that  each  pair  of  contrasting  characters 


SEGREGATION  AND   PURITY   OF   GAMETES  545 

behaved  independently  of  each  other,  l)iit  all  possible  combina- 
tions of  them  could  be  obtained.  The  F2  generation  of  seeds 
contained  smooth  and  yellow,  wrinkled  and  yellow,  smooth  and 
green,  and  wrinkled  and  green  seeds,  and  each  kind  of  seeds 
occurred  in  a  definite  proportion  of  about  9  smooth  and  yel- 
low: 3  wrinkled  and  yellow:  3  smooth  and  green:  1  wrinkled 
and  green.  The  wrinkled  green  seeds  were  pure  recessives  and 
bred  true,  and  1  out  of  9  of  the  smooth  yellow  seeds  was  a  pure 
dominant  and  thus  l)red  true.  All  of  the  other  seeds  were  not 
pure  and  various  combinations  again  occurred  in  their  offspring. 
The  combinations  and  the  number  of  individuals  in  each  com- 
bination that  occurred  in  the  F2  generation  were  in  accord  with 
mathematical  laws  governing  combinations.  Representing  the 
dominants,  smooth  and  yellow,  by  large  8  and  large  Y,  and  the 
recessives,  wrinkled  and  green,  by  small  w  and  small  g,  the  com- 
binations of  S  and  w  are  SS  -{-  2  Sw  -\-  ww,  and  the  combina- 
tions of  Y  and  g  are  YY  +  2  Yg  +  gg.  These  combinations  are 
simply  the  pure  dominants,  impure  dominants,  and  recessives  in 
the  ratio  of  1:2:1  which  occurs  when  a  pair  of  contrasting 
characters  is  considered  separately.  Now  (SS  +  2  Sw  +  ww) 
{YY  -\-2  Yg  +  gg)  =  SSYY  +  2  SYYw  +  YYww  +  2  YgSS  + 
4  YSwg  +  2  Ygwiv  +  SSgg  +  2  ggSw  +  ggww,  which  are  the  dif- 
ferent combinations  and  the  relative  numbers  of  individuals 
in  each  combination  obtained  when  two  pairs  of  contrasting 
characters  were  considered  in  relation  to  each  other.  Since  the 
dominants  obscure  the  recessives,  the  apparent  combinations 
with  the  relative  number  of  individuals  in  each  are  9  dominants, 
3  individuals  with  dominant  yellow  and  recessive  wrinkled,  3 
individuals  with  dominant  smooth  and  recessive  green,  and  1 
individual  with  recessive  wrinkled  and  green.  The  individuals 
having  the  constitution  ]'F.S*S,  as  represented  in  above  formula, 
are  pure  dominants,  the  individuals  having  the  constitution 
wiogg  are  pure  recessive,  while  the  others  are  not  pure.  Thus 
the  laws  of  mathematics  afford  a  way  of  expressing  what  Mendel 
discovered  concerning  the  behavior  of  characters  in  inheritance. 

He  crossed  Peas  having  smooth  yellow  seeds  and  gray-brown 
seed  coats  with  Peas  having  wrinkled  green  seeds  with  white 
seed  coats,  thus  employing  three  pairs  of  contrasting  characters. 
He  found  also  in  this  case  that  the  pairs  of  contrasting  characters 
behaved  independently  of  each  other,  and  that  the  combinations 


546  HEREDITY 

in  the  Fo  generation  were  of  many  kinds.  The  combinations 
in  this  case  also  agreed  quite  well  with  the  mathematical  laws 
of  combination,  when  a  large  number  of  the  F2  individuals 
were  taken  into  account.  The  kinds  of  combinations  and  their 
proportions  follow  quite  well  the  general  algebraic  formula 
(a  +  h)'\  in  which  n  represents  the  number  of  characters  in- 
volved. Thus  (a  +  hy  expanded  gives  a^  -\-  2  ah  -\- l)^  which  is 
in  accord  with  the  1:2:1  ratio,  the  ratio  expressing  the  inheri- 
tance of  two  contrasting  characters.  The  formula  (a  +  6)*  gives 
the  combinations  when  plants  are  crossed  that  have  two  pairs 
of  contrasting  characters.  Of  course  the  results  obtained  scarcely 
ever  exactly  agree  with  the  mathematical  formula,  and  the  more 
individuals  taken  into  account,  the  closer  the  agreement. 

As  a  result  of  his  work  with  a  number  of  pairs  of  characters, 
Mendel  showed  that  by  means  of  repeated  artificial  fertilization, 
the  constant  characters  of  different  varieties  of  plants  may  be 
obtained  in  all  of  the  associations  which  are  possible  according 
to  the  mathematical  laws  of  combination.  This  means  that,  by 
crossing  in  a  certain  way,  the  desirable  characters  of  different 
varieties  may  be  brought  together,  and  thereby  plants  of  a  more 
desirable  type  produced. 

Mendel's  Law.  —  Mendel's  discoveries  and  conclusions  con- 
cerning the  way  parental  characters  appear  in  hybrids  and  their 
offspring  constitute  Mendel's  law,  and  his  discoveries  may  be  sum- 
marized as  follows:  (1)  characters  are  of  two  kinds,  dominant  and 
recessive;  (2)  characters  do  not  blend  but  behave  as  units  and 
separate  completely  from  one  another;  (3)  gametes  are,  there- 
fore, pure,  never  containing  genes  for  both  of  a  pair  of  contrast- 
ing characters ;  (4)  the  offspring  of  a  hj'brid  consist  of  dominants 
and  recessives  in  the  ratio  of  three  dominants  to  one  recessive, 
and  the  recessives  and  one-third  of  the  dominants  breed  true, 
while  two-thirds  of  the  dominants  breed  as  hybrids,  producing 
offspring  consisting  of  dominants  and  recessives  and  again  in 
the  ratio  3:1;  (5)  when  any  number  of  pairs  of  characters 
are  considered,  each  pair  behaves  independently,  and  all  com- 
binations of  characters  according  to  the  mathematical  laws  of 
combination  can  be  obtained. 

After  eight  years  of  work,  Mendel  published  an  account  of 
his  remarkable  discoveries,  but  unfortunately  his  publication 
remained  unnoticed  until  1900,  thirty-five  years  after  its  pub- 


INVESTIGATIONS  SINCE  MENDEL  547 

lication  and  sixteen  years  after  Mendel's  death.  In  1900  his 
paper  was  discovered  simultaneously  by  three  students  of 
genetics,  Correns,  De  Vries,  and  Tschermak,  who  recognized 
its  importance.  Since  that  time  Mendel's  law  has  formed  the 
basis  of  all  work  in  genetics. 

The  Value  of  Mendel's  Discoveries.  —  Mendel's  discoveries 
have  completely  revised  our  methods  of  investigating  and  ideas 
concerning  heredity.  First,  Mendel's  discoveries  have  im- 
pressed upon  us  the  value  of  pedigree  cultures  in  investigating 
problems  of  heredity.  Second,  they  afford  us  laws  concerning 
the  appearance  of  characters  in  the  offspring,  whereby  we  know 
what  to  expect  and  can  thereby  interpret  results  which  were 
previously  a  medley  and  not  understandable.  Third,  knowing 
how  characters  behave  in  the  offspring  when  plants  or  animals 
are  crossed,  we  can  start  in  our  crossing  work  with  definite 
results  to  be  obtained  in  mind  and  also  plan  a  definite  method 
of  procedure  to  obtain  the  desired  results.  Fourth,  owing  to 
the  discovery  of  the  segregation  of  characters,  we  now  know 
that,  in  the  second  generation  of  hybrids,  individuals  that  are 
perfectly  pure  occur  in  definite  proportions  and  that  purity  of 
plants  and  animals  in  respect  to  a  character  does  not  depend 
upon  a  long  series  of  selections  as  was  formerly  the  notion. 
Fifth,  the  law  of  dominance  explains  why  plants  or  animals 
impure  in  respect  to  a  character  may  appear  just  as  pure  as 
pure  individuals.  Sixth,  knowing  that  some  characters  are 
recessive  and  are  entirely  obscured  by  the  contrasting  domi- 
nant characters,  we  can  now  explain  the  appearance  in  the 
offspring  of  a  character  which  did  not  appear  in  the  parents  or 
even  for  generations  back,  and  in  this  way  account  for  many  of 
the  variations  in  offspring,  such  as  talented  offspring  of  medi- 
ocre parents,  blue-eyed  children  of  brown-eyed  parents,  bad 
sons  of  pious  preachers,  rust-resistant  plants  of  plants  suscep- 
tible to  rust,  and  so  on.  Seventh,  Mendel's  discovery  that  pairs 
of  contrasting  characters  behave  independently  of  each  other 
but  may  be  combined  in  various  ways  makes  it  possible  for  us  to 
improve  plants  and  animals  by  breeding  them  in  such  a  way  as 
to  bring  the  desirable  characters  of  different  varieties  together 
in  one  individual. 

Investigations  since  Mendel.  —  Since  the  discovery  of  Men- 
del's  paper,  numerous  investigators   have   been   applying   and 


548 


HEREDITY 


Fi(i.  477.  —  Mcinlilisiu  lUuiuustratc'd  in  the  iulu'ritauce  uf  starchy  and 
non-starchy  endosperm  in  Corn.  In  the  top  row,  the  ear  c  shows  the  immedi- 
ate result  obtained  when  the  starchy  parent  (a)  and  non-starchy  parent  (b) 
are  crossed.  It  is  evident  that  starchness  is  completely  dominant,  d,  an 
ear  with  F2  kernels  resulting  from  the  cross,  showing  segregation  of  starchiness 
and  non-starchiness.  Lower  row,  ears  of  plants  grown  from  kernels  of  d.  e,  f, 
and  g,  result  from  planting  starchy  seeds.  One  ear  out  of  the  three  is  pure 
starchy,  h,  result  from  planting  non-starchy  kernels,  showing  that  the  non- 
starchy  kernels  were  pure  for  the  recessive  character.     After  East. 


INVESTIGATIONS  SINCE   IMENDP:L 


549 


testing  out  MoiuleliKin.  In  both  plants  and  animals  numerous 
pairs  of  characters  have  been  found  to  behave  in  accordance 
with  Mendelism.  In  plants  alone  more  than  100  pairs  of  char- 
acters of  various  kinds  have  been  found  to  behave  according 
to  the  Mendelian  conception.     Among  plants,  color  and  shape  of 


I         n 


Fi(i.  478.  —  Mendelism  demonstrated  in  the  inheritance  of  color  in  the 
endosperm  of  Corn,  c,  ear  bearing  the  F,  kernels  of  the  cross  between  a 
(white  endosperm)  and  b  (yellow  endosperm),  showing  dominance  of  yellow. 
d,  ear  bearmg  F^  kernels  of  the  cross,  showing  segregation  of  color.     Micv  East . 

flowers;  color,  shape,  size,  and  quality  of  fruit  and  seeds  {Figs. 
477  and  478);  time  required  to  mature;  resistance  to  disease, 
drought,  and  cold;  and  many  others  have  been  found  to  follow 
Mendel's  law.  Among  live  stock  Mendelism  applies  to  numerous 
characters,  such  as  the  presence  or  absence  of  horns  in  cattle,  the 
color  of  the  hair  in  cattle  and  horses,  the  character  of  the  comb 


550 


HEREDITY 


and  feathers  in  chickens,  etc.  In  man  many  characters,  among 
which  are  insanitj^  and  susceptibihty  to  tuberculosis,  are  known 
to  behave  according  to  Mendel's  law,  and  Eugenics,  which  has 
to  do  with  applying  the  lavv's  of  heredity  in  a  way  to  produce  a 
healthier  and  a  more  efficient  race  of  men,  has  its  chief  sup- 
port in  Mendelism. 

Mendel's  discoveries  have  already  enabled  us  to  make  some 
notable  achievements  in  the  way  of  improving  plants  and  ani- 
mals. By  working  according  to  the  Mendehan  conception, 
many  desirable  varieties  of  the  cereals,   much  more  desirable 


Fig.  479.  —  Height  of  plants  in  the  Fo  generation  of  'Tom  Thumb  Pop 
Corn  (a  dwarf  Corn)  crossed  with  Missouri  Dent  (a  large  Corn).  The  plant 
at  the  extreme  right  is  similar  in  height  to  the  dwarf  parent,  while  the  one  at 
the  extreme  left  is  similar  in  height  to  the  Missouri  Dent.     After  Emerson  and 

East. 


ornamental  plants,  and  various  kinds  of  better  fruits  have  been 
developed. 

Of  course  in  the  extended  investigations  in  Genetics  since 
1900,  many  situations  have  arisen  that  Mendel  did  not  meet 
Cases  have  arisen  in  which  the  Mendelian  behavior  can  be 
explained  better  by  assuming  that  pairs  of  contrasting  characters 
are  due  to  the  presence  and  absence  of  certain  factors  and  not  to 
dominant  and  recessive  factors.  According  to  the  latter  hypothe- 
sis, the  tallness  of  the  tall  variety  of  Peas  is  due  to  the  presence 
of  a  factor  for  tallness,  while  dwarfness  in  the  dwarf  variety  is 


INVESTIGATIONS   SINCE   MENDEL 


due  to  the  absence  of  the  factor  for  tallness.  Since  the  presence 
and  absence  hypothesis  explains  more  cases  than  tlie  dominant 
and  recessive  hypothesis,  it  has  been  generally  accepted. 

Again  Mendel  worked  chiefly  with  qualitative  characters,  which 
have  been  found  to  behave  differently  from  most  quantitative 
characters,  such  as  size  and  weight.  For  example,  in  crossing  large 
and  small  varieties  of  Corn,  the  individuals  of  the  first  hybrid  gcn- 


r 


Fig.  480.  —  Inheritance  of  length  of  ears  in  Corn.  The  ears  Pi  are  ears 
of  the  parent  plant.s  (Tom  Thumb  Pop  Corn  at  the  left  and  Purple  FUnt  Corn 
at  the  right)  chosen  to  represent  the  average  length  of  ears  of  parents.  Notice 
that  the  ear  of  the  Fi  generation  is  intermediate  in  length  between  the  paren- 
tal ears,  while  in  the  F2  generation,  as  shown  by  the  ears  at  the  left  and  right 
of  the  Fi  ear,  the  length  of  ears  range  from  that  of  Tom  Thumb  Poj)  to  that 
of  Purple  Flint.     After  East. 


eration  are  intermediate  in  size  between  the  parents,  the  size  of 
neither  parent  dominating,  and  in  the  second  hybrid  generation 
the  individuals  are  of  various  sizes,  ranging  from  that  of  the 
smaller  to  that  of  the  larger  parent  (Figs.  479,  48O  and  48 1). 
At  first  such  cases  were  considered  striking  exceptions  to  Men- 
del's law.  However,  a  more  careful  study  has  led  to  the  view 
that  quantitative  characters  do  mendelize  but  commonly  depend 
upon  so  many  independent  factors,  each  of  which  is  responsible 


552 


HEREDITY 


for  a  part  of  the  character,  that,  although  they  do  segregate  and 
combine  according  to  MendeHsni,  they  form  so  many  kinds  of 
combinations  and  thus  so  many  kinds  of  individuals  occur  in  the 
second  generation  of  hybrids  that  it  is  difficult  to  detect  Mende- 
lian  ratios.  For  example,  in  the  case  of  crossing  the  tall  and  small 
varieties  of  Corn,  it  is  assumed  that  the  tall  variety  has  a  number 


Fig.  481.  —  Inheritance  of  size  and  beards  in  Wheat.  Parent  types 
(Turkey  X  Bluestem  at  the  left  and  a  hybrid  No.  143  at  the  right)  and  be- 
tween the  parent  types  the  Fi  generation  of  the  Cross.  In  the  Fi  generation 
the  heads  are  somewhat  intermediate  in  size  and  have  short  beards.  After 
Gaines. 


of  factors  for  size  that  are  not  present  in  the  small  variety.  Let 
us  suppose  the  large  variety  has  four  extra  factors  for  size  that 
are  not  present  in  the  small  variety.  These  factors  may  be  rep- 
resented by  A,  B,  C,  and  D.  Since  the  tall  variety  is  pure  for 
its  height,  its  extra  height  over  the  small  variety  is  due  to  the 
presence  oi  AABBCCDD,  and  the  corresponding  constitution  of 
the  small  variety  is  aabhccdd,  in  which  the  small  letters  represent 


INVESTIGATIONS  SINCE   MENDEL  553 

the  absence  of  the  extra  factors  for  size.  If  the  tall  variety  is 
32  inches  taller  than  the  small  variety,  each  of  the  factors  A ,  B, 
C,  and  D  represents  4  inches  in  height.  Now  the  gametes  of 
the  tall  variety  have  A  BCD  in  them,  while  the  corresponding 
constitution  of  the  gametes  of  the  small  variety  is  abed,  and 
the  fertilized  eggs  and  first  generation  of  hybrids  resulting 
from  the  cross  between  these  two  varieties  have  the  formula 
AaBhCcDd.  Now  since  each  factor  for  height  represented  by 
the  large  letters  is  responsible  for  4  inches  of  height,  the  individ- 
uals of  the  first  generation  of  hybrids  should  be  16  inches  taller 
than  the  small  variety  but  16  inches  shorter  than  the  tall  variety. 
They  are  intermediate  in  height  between  the  two  parents.  The 
hybrids  form  gametes  having  the  constitution  ABCD,  aBCD, 
abCD,  abcD,  abed,  AbCD  and  so  on,  involving  all  the  combina- 
tions that  can  be  made  with  the  four  pairs  of  letters.  In  fertili- 
zation all  possible  combinations  of  the  various  kinds  of  gametes 
can  take  place,  and  consequently  individuals  of  eight  various 
sizes  can  occur  in  the  second  generation  of  hybrids.  Thus,  if  a 
gamete  with  a  constitution  abed  unites  with  a  gamete  with  the 
constitution  abcD,  the  resulting  offspring  has  the  constitution 
aabbeedD  and  should  be  4  inches  higher  than  the  smaller  variety 
of  the  parent  generation.  If  a  gamete  with  the  constitution 
ABCD  unites  with  a  gamete  having  the  constitution  abCD,  the 
resulting  offspring,  which  has  the  factors  aAbBCCDD,  should 
be  24  inches  higher  than  the  smaller  variety  or  8  inches  lower 
than  the  taller  variety  of  the  parent  generations.  It  is,  there- 
fore, obvious  that  due  to  the  various  kinds  of  combinations 
that  may  occur  among  the  gametes,  individuals  of  various 
sizes  may  occur  in  the  second  hybrid  generation. 

Another  peculiar  situation  which  has  been  discovered  among 
both  plants  and  animals  may  be  illustrated  by  the  behavior  of 
color  in  the  Andalusian  fowl.  These  fowls  are  what  fanciers 
call  blue,  but  when  they  are  bred  together  the  offspring  con- 
sist of  black,  blue,  and  white  fowls,  and  the  proportion  is  accord- 
ing to  the  Mendehan  ratio  1  :  2  :  1.  The  black  and  white  fowls 
breed  true,  but  the  blues  breed  as  before.  When  black  and 
white  fowls  are  crossed,  blue  fowls  are  obtained.  The  blue  is 
therefore  a  result  of  a  heterozygous  condition  in  which  the 
factor  for  black  is  combined  with  a  factor  for  white.  In  this 
case  the  hybrids  may  be  regarded  as  having  a  different  character 


554  HEREDITY 

from  that  of  either  parent.  A  similar  situation  has  been  dis- 
covered in  connection  with  the  breeding  of  Sweet  Peas.  Cer- 
tain white-flowered  varieties  of  Sweet  Peas  when  crossed  produce 
red-flowered  offspring.  There  are  still  a  number  of  other  situa- 
tions that  Mendel  did  meet  in  his  experiments. 

To  more  recent  investigators  we  are  also  indebted  for  some 
present  conceptions  of  the  inheritable  constitution  of  organisms. 
Johannsen  of  Copenhagen,  Denmark,  who  is  responsible  for  the 
pure  line  theory,  has  done  much  to  establish  the  theory  that 
inheritance  is  due  to  the  reappearance  of  the  same  organization 
of  protoplasm  with  reference  to  genes  or  character  units  in  suc- 
cessive generations  and  not  to  the  transmission  of  external 
characters.  The  sum  total  of  all  the  genes  in  a  gamete  or  fertil- 
ized egg  Johannsen  calls  a  genotype,  while  an  organism  consid- 
ered as  to  its  appearance  he  calls  a  phenotype.  Organisms  may 
be  alike  genotypically,  that  is,  alike  as  to  genes  but  be  very 
different  phenotypically.  For  example,  two  Corn  plants  may 
be  exactly  alike  in  their  genes  for  size,  yield,  etc.,  but  due  to  a 
difference  in  environment  differ  greatly  in  these  features.  On 
the  other  hand,  organisms  may  be  different  genotypically  but 
be  very  similar  phenotypically. 

Segregation  and  the  Reduction  Division.  — •  As  previously 
stated,  the  purity  of  gametes  and  the  segregation  of  characters 
depend  upon  the  separation  of  the  genes  for  contrasting  char- 
acters. A  plant  that  is  a  hybrid  for  tallness  and  dwarfness  can 
not  have  gametes  pure  for  tallness  and  dwarfness,  unless  the 
genes  for  these  contrasting  characters  are  separated  so  as  to 
appear  in  different  cells.  The  reduction  division,  which  always 
precedes  the  formation  of  gametes  in  both  plants  and  animals, 
affords  a  mechanism  by  which  genes  may  be  segregated.  The 
constant  occurrence  of  the  reduction  division  and  also  the  fact 
that  it  is  the  division  in  which  chromosomes  are  separated, 
suggest  that  it  has  some  vital  connection  with  heredity. 

It  is  generally  believed  that  the  genes  are  associated  with  the 
chromatin  of  the  nucleus  and  are,  therefore,  distributed  with  the 
chromosomes  to  new  cells  during  cell  division.  The  chromatin 
of  a  plant  or  animal  consists  of  the  chromatin  contributed  by 
each  of  its  parents.  At  each  cell  division  this  chromatin  is 
organized  into  a  definite  number  of  chromosomes,  and  there  is 
considerable  evidence  that  the  chromatin  of  each  of  the  parents 


INVESTIGATIONS  SINCE   MENDEL 


555 


of  the  plant  or  animal  whose  cell  is  dividing  organizes  separatdy 
into  chromosomes,  thus  one-half  of  the  number  of  chromosomes 
being  composed  of  father  chromatin  and  the  other  half  being  com- 
posed of  mother  chromatin.  This  means  that  the  chromosomes 
contributed  to  the  offspring  by  each  of  the  parents  maintain 
their  individuality  in  the  offspring.  In  vegetative  cell  division 
each  chromosome  splits  longitudinally,  and  to  each  new  nucleus 
there  is  contributed  a  half  of  each  chromosome.     It  is  obvious 


Fig.  482.  —  A  diagram  illustrating  the  behavior  of  chromatin  in  the  reduc- 
tion division.  For  convenience  the  chromatin  contributed  by  the  father  of 
the  plant,  the  division  of  whose  cell  the  diagram  illustrates,  is  shown  black 
and  the  chromatin  contributed  by  the  mother  plant  is  shown  white.  In  the 
upper  line,  organization  of  the  chromosomes  and  their  pairing,  each  pair  con- 
sisting of  one  father  and  one  mother  chromosome;  in  the  lower  line,  the  dis- 
tribution of  the  chromosomes  in  the  formation  of  the  daughter  nuclei.  In 
this  case  one  of  the  daughter  nuclei  receives  one  father  and  three  mother 
chromosomes,  while  the  other  daughter  nucleus  receives  one  mother  and  three 
father  chromosomes,  but  this  is  only  one  of  a  number  of  ways  of  distributing 
the  chromosomes. 

that  the  vegetative  cell  division  tends  to  distribute  the  chromatin 
from  both  parents  equally  to  the  new  nuclei.  But  in  the  reduc- 
tion division,  as  shown  in  Figure  4^2,  whole  chromosomes  and 
not  halves  are  contributed  to  each  new  nucleus,  and  conse- 
quently the  new  nuclei  resulting  from  the  reduction  division 
receive  only  half  as  many  chromosomes  as  the  mother  cell  con- 
tained. In  the  reduction  division  the  chromosomes  contributed 
to  the  daughter  nuclei  may  be  only  those  of  the  mother  parent  or 
only  those  of  the  father  parent,  in  which  case  the  daughter  nuclei 


556  HEREDITY 

receive  only  the  genes  of  one  of  the  parents.  On  the  other 
hand,  the  daughter  nuclei  may  receive  chromosomes  of  both 
parents  and  in  different  proportions  in  different  divisions.  Again 
cytological  studies  of  the  reduction  division  show  that  there 
is  a  pairing  of  chromosomes  previous  to  their  separation,  and 
there  is  evidence  that  each  pair  consists  of  a  father  and  a. 
mother  chromosome.  Now,  if  we  assume  that  chromosomes 
pairing  carry  genes  for  contrasting  characters,  then  the  separa- 
tion and  distribution  of  the  members  of  each  pair  to  different 
daughter  nuclei  should  result  in  the  segregation  of  genes  for 
contrasting  characters  and  in  the  production  of  pure  gametes. 
The  trouble  with  this  assumption  is  that  a  plant  or  animal  has 
so  many  more  pairs  of  contrasting  characters  than  chromo- 
somes, that  it  is  difficult  to  explain  the  numerous  combinations 
that  occur  when  many  pairs  of  contrasting  characters  are  taken 
into  account.  Despite  the  fact  that  there  are  some  things 
about  segregation  we  are  unable  to  explain  by  the  mechanism 
of  reduction  division,  it  is  generally  believed  that  the  two  phe- 
nomena are  vitally  related. 

The  Mendelian  Ratio  and  the  Combinations  of  Gametes.  — 
It  is  possible  to  account  for  the  Mendelian  ratio  1:2:1  by 
taking  into  account  the  probable  combinations  that  may  occur 
among  gametes  during  fertilization.  A  hybrid  forms  two  kinds 
of  gametes  equal  in  number  in  respect  to  a  pair  of  contrasting 
characters.  One  kind  of  sperms  and  eggs  may  be  represented 
Ijy  A  and  the  other  by  B.  Now  the  probable  combinations 
between  the  two  kinds  of  sperms  and  two  kinds  of  eggs  in  the 

self-fertilization  of  a  hybrid  are  represented  by  |    X,    I  •     There 

A"^  ^B 
are  two  chances  for  A  and  B  to  unite  to  one  chance  for  A  to 
unite  with  A  or  B  to  unite  with  B.  The  probable  combinations 
and  their  ratios  are,  therefore,  AA  :2AB  :  BB  or  1:2:1. 
If  the  factors  represented  by  either  A  or  B  are  dominant,  then 
3  :  1  is  the  ratio  of  the  dominant  to  the  recessive  offspring. 


CHAPTER   XXIV 
PLANT   BREEDING 

Plant  breeding  has  to  do  with  the  improvement  of  old  plants 
and  the  securing  of  new  ones.  Many  and  various  are  the  aims 
of  plant  breeding.  The  object  may  be  to  improve  the  yield, 
increase  the  resistance  to  drought  or  disease,  shorten  the  period 
of  development,  or  secure  strains  or  varieties  with  new  chara- 
ters.  The  two  important  methods  used  are  selection  and  hy- 
bridization. 

In  connection  with  plant  breeding,  the  discoveries  of  De  Vries 
and  Mendel  have  proven  to  be  of  inestimable  value.  The  dis- 
coveries of  De  Vries  have  resulted  in  a  better  understanding 
of  the  nature  of  variations  and  has  enabled  us  to  improve  plants 
by  selection  much  more  efficiently.  The  introduction  of  Men- 
del's methods  of  investigation  and  his  discoveries  concerning 
the  behavior  of  characters  in  hybrid  offspring  afford  a  scientific 
foundation  to  the  improvement  of  plants  by  hybridization.  As 
a  result  of  Mendel's  contributions,  we  now  know  much  more 
about  how  to  proceed,  what  to  expect,  and  how  to  interpret  the 
results  obtained  in  hybridizing. 

Selection.  —  Selection  takes  advantage  of  variations.  Vari- 
ations, as  previously  noted,  are  not  only  due  to  differences  in 
gametes  and  their  combinations  in  fertilization,  but  also  to 
differences  in  temperature,  moisture,  soil  conditions,  and  other 
environmental  factors.  But  only  those  variations  due  to  factors 
which  can  be  transmitted  by  the  gametes  are  inheritable  through 
the  seed.  A  Corn  plant  may  be  larger  or  bear  larger  ears  than 
the  ordinary  type  because  of  especially  favorable  conditions. 
It  may  stand  alone  in  the  hill,  thus  having  all  of  the  water  and 
mineral  supply  to  itself,  or  it  may  be  growing  on  ground  more 
heavily  fertilized.  This  increased  size,  due  to  external  condi- 
tions and  not  to  any  special  factors  in  the  cells  of  the  plant, 
is  not  transmitted  to  the  offspring,  and  the  embryos  in  the 
kernels  of  this  especially  favored  plant  may  have  inherited  no 


558 


PLANT  BREEDING 


more  for  size  than  the  embryos  developed  by  a  much  smaller 
plant  which  has  grown  in  less  favorable  conditions.  To  com- 
pare plants  as  to  what  is  really  in  their  cells,  the  conditions 
under  which  the  plants  were  grown  must  be  considered.  It  is 
for  this  reason  that  it  is  much  better  to  select  seed  Corn  from  the 
field  than  to  select  it  from  the  crib;  for  by  the  first  method  the 
conditions  under  which  the  plants  developed 
can  be  considered  and  their  genetic  consti- 
tution better  estimated. 

In  addition  to  Johannsen's  investigations 
of  inheritance  in  Beans  and  a  number  of 
other  plants,  there  are  many  other  experi- 
ments that  tend  to  establish  the  fact  that 
little  is  gained  by  selections  in  pure  lines, 
unless  mutations  happen  to  occur.  For 
example,  as  previously  cited  a  long  series  of 
selections  in  a  pure  line  of  Oats  to  increase 
the  yield  secured  no  results.  Likewise  selec- 
tions in  pure  lines  of  Wheat  to  intensity  and 
fix  a  desirable  variation  have  given  no  re- 
sults. In  each  generation  plants  having  the 
desirable  variation  were  selected  for  seed, 
but  selections  carried  on  this  way  for  a 
number  of  generations  did  not  increase  the 
average  of  the  variation. 

In  vegetative  propagation,  where  the  pro- 
geny does  not  develop  from  seed  but  grows 
from  vegetative  structures  taken  from  the 
parent,  some  variations,  such  as  bud  sports, 
that  are  often  not  inheritable  through  seed 
can  be  perpetuated.     Such  is  true  in  such 
plants  as  Strawberries,  which  are  propagated 
by  runners,  and   in   fruit    trees,  which   are 
propagated   by   grafting.      But   modifications   that   are   simply 
responses  to  some  peculiarity  in  the  environment  are  not  per- 
petuated even  by  vegetative  propagation. 

Although  selection  often  fails  to  produce  the  desired  results, 
nevertheless,  selection  is  one  of  the  chief  means  of  improving 
plants.  A  mass  of  plants,  such  as  a  field  of  grain,  is  a  mixture 
of  individuals  most  of  which  are  heterozygous  for  one  or  more 


Fig.  483.  — Heads 
of  Wheat,  showing 
improvement  in  size 
as  a  result  of  a  five- 
year  selection  in 
which  the  plants 
bearing  the  largest 
heads  were  selected 
each  year  for  seed. 
Redrawn  from  De 
Vries. 


MASS   CULTURE 


559 


characters.  By  selecting,  through  a  number  of  generations,  the 
most  desirable  individuals  for  seed,  eventually  individuals  that  are 
homozygous  and  breed  true  to  the  desirable  character  may  be 
secured,  and  a  better  race  of  plants  thereby  established.  Muta- 
tions also  afford  considerable  opportunity  to  hnprove  plants  by 
selection.  In  case  individuals  appear  that  are  more  desirable  due 
to  a  mutation,  a  more  desir- 
able race  of  individuals  is 
immediately  secured  by 
selecting  and  propagating 
these  mutants.  Through 
selection,  races  of  plants  much 
more  desirable  than  the  ordi- 
nary types  from  which  the 
plants  were  selected  have 
been  produced.  Grains,  for- 
age crops,  Strawberries, 
Blackberries,  Melons,  fruit 
trees,  etc.,  have  been  im- 
proved by  selecting  and  prop- 
agating those  plants  having 
more  desirable  features  than 
the  ordinary  types.  In  this 
way  plants  have  been  im- 
proved in  yield  (Figs.  483  and 
484),  ability  to  resist  drought 
and  disease,  length  of  period 

required  for  maturing  (Fig.  480),  and  many  other  ways.  There 
are  two  methods  used  in  improving  plants  by  selection,  the  mass 
culture  and  pedigree  culture. 

Mass  Culture.  —  This  is  the  oldest  method  of  plant  breeding. 
This  method  employs  large  masses  or  fields  of  plants,  known  as 
mass  cultures,  from  which  to  select.  For  example,  in  applying 
this  method  to  the  breeding  of  small  grains,  the  plant  breeder, 
desiring  to  produce  a  better  yielding  race,  goes  through  the  field 
and  selects  those  plants  with  heads  having  the  largest  number  of 
grains.  From  the  next  j^ear's  crop  grown  from  the  seed  of  the 
plants  selected  the  previous  year,  the  best  yielding  plants  are 
again  selected  for  seed,  and  year  after  year  he  continues  to  select 
until  a  race  more  or  less  constant  for  high  yield  is  obtained. 


Fig.  484. — Heads  of  Timothy,  show- 
ing improvement  by  selection.  After 
Hays. 


560 


PLANT  BREEDING 


This  method  of  selection  is  productive  of  good  results,  but 
has  some  disadvantages.  It  requires  much  time  and  labor  as 
well  as  the  use  of  much  ground.  Since  each  crop  is  grown 
from  seed  furnished  by  many  plants  selected  the  previous  year, 
the  progeny  of  many  plants  are  involved  and  the  yield  of  a  crop 


Fig.  485.  —  Dakota  Amber  Sargo,  a  strain  that  matures  much  earher  and 
is  more  drought-resistant  than  the  South  Dakota  No.  341  from  which  this 
new  strain  was  produced  by  selection.     After  DiUman. 

is  the  average  yield  of  the  descendants  of  many  plants  varying 
in  capacity  and  heritage  for  high  yield.  Many  of  the  plants 
in  the  selection  are  likely  to  be  heterozygous  for  the  character 
and  consequently  will  not  breed  true.  Races  obtained  by  this 
method  of  selection  usually  lose  their  desirable  features  unless 
selection  is  continued. 

Pedigree  Culture.  —  The  value  of  pedigree  cultures  was  well 
demonstrated  Ijy  De  Vries  and  Mendel.  In  the  m_ethod  of 
selection  by  pedigree  cultures,  a  single  plant  is  selected,  and 
from  its  progeny,  which  are  carefully  guarded,  the  best  indi- 


HYBRIDIZATION  561 

viduals  are  selected.  After  continuing  the  selection  for  a  few 
generations,  a  race  with  a  certain  standard  and  steadiness  is 
obtained.  The  race  obtained  by  this  method  of  selection  is 
the  progeny  of  a  single  individual,  and  its  desirable  features  are 
more  stable  than  in  most  races  secured  by  mass  selection.  This 
method  also  requires  less  labor  and  usually  less  time  than  the 
method  of  mass  culture.  After  the  race  is  secured  by  pedigree 
culture,  it  is  usually  tested  in  mass  culture  to  see  how  it  behaves 
when  grown  in  masses  under  ordinary  field  conditions. 

Selection  of  Mutants.  —  Many  valuable  races  of  plants  have 
been  discovered  accidentally  and  apparently  have  arisen  sud- 
denly. The  Fultz  Wheat  comes  from  a  few  plants  which  were 
accidentally  discovered  growing  in  a  field  of  Lancaster  Red. 
These  few  plants,  which  were  smoother  and  had  more  beautiful 
heads  than  the  Lancaster  Red,  were  saved  for  seed,  and  from 
these  seeds  the  well-known  and  valuable  race  of  Fultz  Wheat 
originated.  The  Gold  Coin  Wheat  was  accidentally  found 
growing  in  a  field  of  Mediterranean  Wheat.  There  are  a  num- 
ber of  varieties  of  Wheat,  Oats,  Barley,  and  Rye  which  appar- 
ently originated  in  a  similar  way. 

Many  or  all  of  the  different  cultivated  varieties  of  Dewber- 
ries were  accidentally  found  growing  wild  and  were  selected 
because  they  showed  some  desirable  features  not  possessed  by 
the  ordinary  type  of  wild  Dewberries.  Some  may  be  hybrids, 
while  others  are  most  likel}"  mutants. 

In  woody  plants,  such  as  fruit  trees,  the  selection  of  vegeta- 
tive mutations  known  as  bud  sports,  in  which  a  branch  may 
produce  a  type  of  fruit  different  from  the  fruit  produced  by 
other  branches,  often  leads  to  the  establishment  of  new  varieties. 
By  propagating  these  special  branches  by  grafting,  a  different 
type  of  tree  may  be  obtained.  The  Nectarine  has  already  been 
mentioned  as  arising  in  this  way,  and  there  still  are  other  ex- 
amples among  Peaches,  Apples,  and  other  fruits.  Greening 
Apples  often  have  branches  bearing  Russet  Apples,  and  Russet 
Apples  often  have  branches  bearing  Greening  Apples.  There 
are,  therefore,  many  instances  in  which  selection  has  not  only 
resulted  in  the  securing  of  better  grains,  vegetables,  fruits,  and 
ornamental  plants,  but  also  in  new  types. 

Hybridization.  —  The  advantage  of  hyl)ridization  is  that  by 
crossing  one  can  combine  in  the  offspring  the  different  desirable 


562  PLANT   BREEDING 

features  of  the  two  plants  used  in  the  cross.  Since  Mendel's 
discoveries  have  furnished  principles  that  make  it  possible  to 
interpret  the  behavior  of  hybrids,  one  can  proceed  with  consid- 
erable certainty.  As  to  just  how  the  factors  introduced  by  the 
sperm  and  egg  will  manifest  themselves  in  the  offspring  resulting 
from  a  cross  is  not  known  until  the  offspring  appear.  The  char- 
acters, whether  they  blend  or  behave  as  dominants  and  recessives, 
are  identified  only  by  observations  of  the  hybrid  generations.  The 
hybrid  ma}'  be  like  one  parent  in  some  features  and  like  the  other 
parent  in  other  features,  or  in  size  and  some  other  characters  it 
may  be  intermediate  betwsen  the  two  parents.  According  to 
Mendel's  law,  we  can  expect  three  kinds  of  individuals  in  the 
Fo  generation  when  one  pair  of  contrasting  characters  is  con- 
sidered, and  that  the  pure  dominants  and  pure  recessives  will 
breed  true  whether  one  or  many  pairs  of  characters  are  taken 
into  account.  The  pure  dominants  and  pure  recessives  can  be 
identified  by  further  breeding,  and  if  they  prove  to  be  more 
desirable  than  the  varieties  used  in  the  cross,  then  by  propa- 
gating them  a  more  desirable  race  or  variety  is  established. 
In  case  one  wishes  to  bring  together  in  one  individual  a  number 
of  desirable  characters,  some  of  which  are  present  in  one  variety 
and  some  in  another,  the  breeding  process  to  obtain  the  indi- 
viduals pure  for  these  characters  is  complex,  as  was  shown  in 
the  discussion  of  Mendelism,  and  the  more  factors  involved,  the 
more  complex  is  the  process.  But  Mendel's  law  points  the 
way  of  procedure,  and  it  is  possible  for  the  patient  plant  breeder 
to  so  manipulate  the  breeding  through  a  number  of  generations 
as  to  finally  obtain  a  combination  of  the  desirable  characters  in 
an  individual  that  will  breed  true.  The  desired  individual  hav- 
ing been  secured,  the  new  race  or  variety  is  practically  estab- 
lished. Much  has  been  accomplished  in  improving  plants 
through  hybridization.  For  example,  in  this  way  a  much  more 
desirable  race  of  Wheat  has  been  obtained  in  England.  One 
variety  of  English  Wheat,  yielding  well  but  producing  a  poor 
grade  of  flour,  was  crossed  with  a  variety  of  Canadian  Wheat, 
which  produces  a  good  grade  of  flour  but  does  not  yield  so  well 
in  the  English  climate  as  the  English  variety.  The  plant  breeder 
finally  succeeded  in  getting  a  race  having  the  desirable  features 
of  producing  good  flour  and  jaelding  well  in  the  English  climate 
By  crossing  Wheat,   having  some  desirable  qualities  but  sus- 


HYBRIDIZATION 


563 


ccptiblo  to  Rust,  with  Wheat,  iniiuuno  to  Rust  but  less  desirable 
in  other  features,  a  type  of  Wheat  having  the  Rust  resistance 
of  one  parent  and  the  desirable  features  of  the  other  has  been 
obtained.  Cotton  producing  longer  and  better  lint  has  l^een 
obtained  by  crossing  the  Sea  Island  Cotton  with  the  Upland 
Cottons.  There  are  many  instances  in  which  more  desirable 
races  have  ])een  secured  through  hybridization. 


Fig.  48G.  —  The  effect  of  three  degrees  of  relationship  in  breeding  Corn. 
Nos.  3  and  4  are  pure  strains  from  seed-stock  inbred  for  three  years.  No.  2 
is  from  a  closc-fertihzed  seed-stock,  the  plants  each  year  being  fertilized  with 
pollen  from  sister  plants  grown  from  the  same  ear.  No.  1  is  from  seed-stock 
that  has  been  cross-fertilized  for  three  years.     After  Montgomery. 

The  greatest  advantage  arising  from  hybridization  is  among 
plants  propagated  by  vegetative  methods,  as  by  tubers,  bulbs, 
cuttings,  layering,  grafting,  etc.;  for  in  these  cases  the  progeny 
is  simply  a  continuation  of  the  hybrid  individual  and  not  the 
result  of  the  fusion  of  gametes.  Many  berries,  vegetables,  fruit 
trees,  and  ornamental  plants  are  hybrids.  By  crossing  different 
kinds  of  Strawberries,  hybrids  more  desirable  than  either  of  the 
parents  have  been  obtained,  and  since  they  propagate  by  run- 


564 


PLANT   BREEDING 


iiers,  the  hybrid  type  is  maintained  generation  after  generation. 
Blackberries,  which  are  propagated  vegetatively,  have  been  im- 
proved by  hybridization.  By  crossing  the  cultivated  Black- 
berry, which  has  a  large  black  fruit,  with  a  small  wild  Blackberry, 
having  a  whitish  or  cream  colored  fruit,  a  Blackberry  having  a 
fruit  large  in  size  and  light  in  color  has  been  obtained.     Many 


^>; 


Fig.  487.  —  Results  of  iubreeditig  and  crossing  on  the  size  of  ears  in  Corn 
Outer  ears,  result  of  inbreeding  one  generation;  middle  ear,  result  in  the  firs 
generation  of  crossing  these  inbred  generations.     After  East. 


of  the  best  Plums  and  other  fruit  trees  are  hybrids,  and  the 
hybrid  characters  are  retained  by  propagating  the  trees  by  graft- 
ing and  budding.  Hybrids  are  also  common  among  Roses,  Car- 
nations, and  other  ornamental  plants. 

Crossing  and  Vigor  of  Offspring.  —  Crossing  usually  results 
in  increased  vigor,  while  self-fertilization  commonly  results  in 


CROSSING   AND   VIGOR  OF  OFFSPRING 


565 


the  loss  of  vigor  in  the  offspring.  Hybrids  are  usually  more 
vigorous  than  their  parents.  Corn  grown  from  seed  resulting 
from  self-fertilization  shows  much  loss  in  vigor  and  consequently 
does  not  yield  so  well  {Figs.  486  and  487).  The  difference  in' 
yield  between  plants  resulting  from  crossing  and  plants  resulting 
from  self-fertiUzation  often  amounts  to  several  bushels  per  acre. 


Fig.  488.  —  Increase  in  size  of  fruits  in  Cucumbers  as  a  result  of  crossing. 
a  and  c  show  size  of  fruits  borne  by  the  parents  and  b,  the  size  of  fruits  borne 
by  the  first  generation  of  the  cross.     After  Halsted. 


Darwin  found  that  Cabbage  plants  obtained  by  crossing  were 
nearly  three  times  the  weight  of  those  obtained  by  self-fertihza- 
tion.  In  Buckwheat  Darwin  obtained  plants  much  taller  and 
about  one-fifth  better  in  yield  by  crossing.  In  Lettuce,  Beets, 
Pumpkins,  Squashes,  Tomatoes,  and  many  other  plants  (Fig. 
488),  it  has  been  shown  that  crossing  produces  more  vigorous 
offspring. 


INDEX 

(The  numbers  with  stars  refer  to  the  pages  on  whioh  the  illustrations  are  given.) 


Absciss  layer,  184. 

Absorption  of  water,  by  seeds,  91;  by 

roots,   158;    factors  that  hinder, 

159;  selective,  159. 
Absorptive  zone,  143,  144*. 
Abundance     and      Distribution     of 

plants,  5. 
Abulilon  Theophrasti,  487. 
Accessory  buds,  206. 
Aceraceae,  487. 
Achene,  60,  79,  60*. 
Achillea  millefolium,  24*. 
Acids,  120;  malic,  285;  oxalic,  285; 

citric,  285;  tartaric,  285. 
Aconite,  482. 
Acorn,  80,  83*. 
Actinomyces  chromogenus,  348,  344*, 

347*. 
Active  buds,  211. 
Adventitious  buds,  206,  207*. 
Adventitious  roots,  140. 
Aecidiospores,  398,  399*. 
Aerial  stems,  172. 
Aerobic  Bacteria,  343. 
Aerotropism,  152. 
After-ripening,  69. 
Agarics,  385,  385*,  386*. 
Agaricus  campestris,  385,  386*. 
Agave,  498,  498*. 
Aggregate  fruits,  79*,  80. 
Agropyron  re  pens,  74. 
Air  chamber,  249. 
Air,  in  the  .soil,  153. 
Air  roots,  162. 
Albugo,  359. 
Albumins,  281. 
Albuminous  seeds,  60. 
Aleuron  layer,  64,  65,  66*,  280*. 
Alfalfa  Dodder,  489 


Alfalfa,  seeds,  74,  87,  74*;  fruits,  82; 

stem,    193*,    194*;   plant,    209*; 

leaf,  238*. 
Algae,  296. 

Blue-green,  297-301. 
Green,  301-318. 
Brown,  318-324. 
Red,  324-329. 
Alkaloids,  281,  283. 
Allelomorphs,   543,   543*;    dominant, 

543;  recessive,  543. 
Alternate  arrangement  of  leaves,  238, 

239*,  240*. 
Alternation  of  generations,  412. 
Amatiita  bulbosa,  384*. 
Amaranthus  albus,  84*. 
Amino  acids,  120. 
Amino  compounds,  281. 
Amygdalase,  285. 
Amygdalin,  282. 
Anabolic  metabolism,  273. 
Anaerol)ic  Bacteria,  343. 
Anaerobic  respiration,  122. 
Analysis  of  seeds,  74. 
Anatomy,    described,    2;    of   leaves, 

242-252;  of  root  tip,  143;  of  the 

older  portion  of  the  root,   147; 

of  stems,  182-203. 
Anatropous  ovule,  463. 
Andreales,  417. 
Angiosperms,    293,    445;    described, 

459;  life  cycle,   469*;  cla.ssifica- 

tion,  471. 
Annuals,  171. 

Annual  rings,  197,  201,  198*,  202*. 
Annular    vessels,    188,    189*,    190*, 

195*. 
Annulus,  386,  429,  385*,  386*,  430*. 
Author,  41,  41*. 
567 


568 


INDEX 


Aiitheridium,  322,  326,  328,  410,  312*, 
316*,  333*,  411*,  419*,  432*. 

Anthoceros,  416,  416*. 

Anthocerotales,  407. 

Anthocyan,  284. 

Antitoxins,  123. 

Apetalae,  473. 

Apetalous  flower,  13,  11*. 

Apical  meristem,  126*,  183*. 

Apophysis,  422. 

Apothecium,  366,  367*,  382*. 

Apple,  flower,  13*;  cyme,  31*;  apple 
pollen,  50;  type  of  fruit,  78;  sec- 
tion of  twig,  199*. 

Apple  Blotch,  404. 

Apple  Rust,  401,  400*,  403*. 

Apple  Scab,  378. 

Archegonia,  409,  410*,  419*,  432*, 
449*. 

Archichlamydeae,  described,  472 ; 
families,  473. 

Arctium  Lappa,  fruits,  86*. 

Arginin,  281. 

Aril,  56,  57*. 

Arisaema  triphyllum,  26*. 

Arrangements,  of  leaves,  238. 

Ascus,  364,  368*,  382*. 

Ascocarp,  365. 

Ascogenous  hyphae,  368,  376,  367*. 

Ascogonium,  368. 

Ascomycetes,  353,  363. 

Ascospore,  364,  366*. 

Asparagus,  160,  271. 

Asparagus  Rust,  403. 

Aspergillus,  375,  375*. 

Associated  i)lants  and  animals,  503. 

Atropine,  283. 

Auricles,  235. 

Auxanometer,  215,  216*. 

Auxograph,  215. 

Auxospore,  332. 

Available  water,  160. 

Awn,  22. 

Bacillus  subtilis,  34'3*. 

Bacteria,  155,  156,  289,  336,  157*; 
described,  341;  coccus  forms, 
341,   342*;    l)acillus  forms,  341, 


342*;  spirillum  forms,' 341,  342*; 

reproduction,  343;  of  decay,  344; 

of  fermentation,   345,   345*;    of 

nitrification,    345,   346*;    patho- 
genic, 346,  347*. 
Balm  of  Gilead,  474. 
Banner,  24. 
Barberry,  396,  399*. 
Bark,  127;  described,  199. 
Basidium,  382,  386*. 
Basidiomycetes,  352;  described,  382. 
Basidiospore,  383,  386*,  399*. 
Basswood,  flower,  10*;  fruits,  83*. 
Bast  fibers,  115,  129,  129*,  193*,  196*, 

198*. 
Bean,  family,  23;  seed,  56*,  57*;  type 

of  seeds,  58;  seedling,  106*;  type 

of  seedling,  107. 
Beech  and  Oak  family,  476;  flowers, 

'476*. 
Beggar-ticks,  fruits,  86*. 
Berberis  vulgaris,  396,  399*. 
Berry,  77,  77*. 
Betulaceae,  475. 
Bidens,  fruit,  86*. 
Biennial,  171. 

Bindweeds,  489*;  described,  490. 
Biometry,  538. 

Birch  family,  475;  flowers,  475*. 
Bird's  nest  Fungus,   described,   391, 

392*. 
Bitter  Rot  of  Apples,  378,  379*. 
Blackberry,  fruit,  79. 
Black  Fungi,  369. 

Black  Knot,  222;  described,  369,  370*. 
Black  leg,  348. 

Black  oak,  section  of  stem,  200*. 
Black-rot  of  cabbage,  347. 
Black-rot  of  Grapes,  378. 
Black-rot  of  Sweet  Potato,  404. 
Black  Rust  of  grain,  described,  396, 

397*,  398*,  399*. 
Black  Walnut,  474. 
Bladder  Plums,  377. 
Blade  of  leaf,    234;   described,   235: 

236*,  237*. 
Bleeding  of  plants,  161. 
Blister  Rust  of  Pines,  described,  402. 


INDEX 


569 


Blotch  of  Apples,  404. 

Blueberries,  489. 

Blue-green  Algae,  described,  297;  re- 
production, 300. 

Blue  Mold,  375;  described,  376. 

Boletus,  387*. 

Boneset,  493. 

Bordeaux  mixture,  359. 

Bordered  pits,  130,  131*. 

Botany,  definition,  1;  derivation,  1. 

Botrychium  Virginianum,  434*;  de- 
scribed, 435. 

Botrydium,  317,  317*. 

Brace  roots,  139. 

Bracket  Fungi,  388,  389*,  390*. 

Bract,  17,  19*. 

Bracted  Plantain,  seeds,  75*. 

Bracts  of  Grass  flowers,  17,  19*,  21*, 
22*. 

Branching,  of  roots,  150;  of  stems, 
167. 

Brand  spores,  394,  394*. 

Brassica  nigra,  483*. 

Brassica  Sinnpislrum,  74. 

Bread  Mold,  360,  362*,  363*. 

Bread  Yeast,  377,  377*. 

Bromelin,  284. 

Brown  Algae,  296;  described,  318. 

Brown  Rot,  368. 

Bryales,  417,  418*. 

Bryophj-^tes,  292;  described,  405; 
groups,  405. 

Buckwheat  family,  478;  flowers,  479*. 

Buckwheat,  flower,  133,  11*;  type  of 
seeds,  59;  fruit,  60*;  plants,  158*. 

Bud,  described,  167,  204*,  205*;  rest- 
ing, 204,  204*;  opening,  205; 
terminal,  206,  206*;  lateral,  206, 
206*;  contents,  207,  213*;  axil- 
lary, 206,  206*;  accessory,  206, 
206*;  adventitious,  206,  206*, 
207*;  formation,  208;  flower, 
207,  210*;  leaf,  207,  210*;  mixeil, 
207,  210*;  active,  211;  dormant, 
211. 

Budding,  225;  described,  231,  232*; 
of  Yeast,  377,  377*. 

Bulb,  179;  described,  181,  181*. 


Bundle  sheath,  252. 
Bunt  or  Stinking  Smut,  393,  395. 
Burdock,  493;  fruits,  86*. 
Buttercup  family,  481. 
Butternut,  474. 

Button    stage    of    Mushroom,    385, 
385*. 

Caffein,  283. 

Calcium,  157,  158*. 

Callus,  224,  225*. 

Calyx,  11,  10*,  11*,  12*. 

Cambium,  126,  126*,  149*,  194*,  195*, 

196*,  198*. 
Cambium  ring,  148. 
Camphor  trees,  482*. 
Campylotropous  ovules,  463. 
Canada  Thistle,   82,    165,   494,   503, 

494*. 
Capillitium,  337,  337*. 
Capillary  water,  153. 
Carbonic  acid,  161. 
Carbohydrates,  277. 

sugars,  277. 

starches,  278,  278*. 

hemi-cellulose,  278,  279*. 
Carica  papaya,  284. 
Cariopsis,  62. 
Carnations,  10. 

Carnivorous  plants,  273,  272*,  273*. 
Carotin,  284. 
Carpels,  15,  14*. 
Carpogonium,  327,  326*,  328*. 
Carpospore,  327,  326*,  328*. 
Caruncle,  57,  57*. 
Caryophyllaceae,  480. 
Castor  Bean,  seed,  57;  seedling,  108; 

use,  486. 
Castor  oil,  279. 
Catabolic  metabolism,  275. 
Catkin,  30,  473,  29*,  473*. 
Cat-tail    family,    495;    flowers,    495, 

495*,  496*. 
Causes  of  variations,  531. 
Cedar  Apples,  401,  401*,  402*,  403*. 
Cedar  Rust,  401,  400*. 
Cells,  39,  96;  position  in  plant  life, 
112;  discovery,  112;  structures, 


570 


INDEX 


112,  113,  114,  114*,  115*;  pres- 
sure within,  118;  size,  112. 

Cell  activity,  processes  involved  in, 
116. 

Cell  division,  described,  124,  125, 
413,  125*. 

CeU  membrane,  114,  115*;  character 
after  death,  119. 

CeU  multiplication,  123. 

CeU  sap,  114. 

CeU  waU,  114;  described,  115. 

CeUular  anatomy,  of  root  tip,  143. 

CeUular  structure,  of  leaves,  246. 

CeUulose,  115,  275,  276. 

Century  Plant,  498,  498*. 

Chaparral,  509. 

Charales,  332,  333*. 

Charafragilis,  333*. 

Chemotropism,  152. 

Chenopodiaceae,  480. 

Cherry  Birch,  flowers,  475*. 

Chestnut,  sprouts,  208*;  disease,  271, 
272*;  nuts,  476. 

Chimera,  230. 

Chlamydomonas,  303,  304,  303*. 

Chlamydospore,  394,  394*. 

Chlorenchyma,  194,  249,  250,  193*, 
251*;  pahsade  tissue,  249,  250*; 
spongy  tissue,  249,  250*,  251*. 

Chlorophyceae,  301. 

ChlorophyU,  115,  294,  115*. 

Chloroplast,  115,  247,  249,  251,  247*. 

Chondrus  crispus,  325*. 

Chondriosomes,  115. 

Chromatin,  114,  114*. 

Chromosome,  124,  125*. 

Chrysanthemums,  10. 

Cicula  maculata,  283,  488. 

Cion,  227,  231,  232*. 

Circinate  vernation,  429. 

Citric  acid,  285. 

Citrus  fruits,  27,  501. 

Cladophora,  312. 

CladophyU,  169,  170*. 

Classes,  291. 

Classification,  of  plants,  291. 

Clavaria,  387*. 

Claviceps  purpurea,  370,  371*. 


Cleft  grafting,  232. 

Cleistothecia,  373,  374*,  375. 

Clematis  vilicella,  186*. 

Climbing  stems,  175. 

Closed  bundles,  191,  191*. 

Closed  Fungi  (Pyrenomycetales),  364; 

described,  369,  370*,  371*. 
Close-pollination,  48. 
Clover  Dodder,  seeds,  75*. 
Club  Mosses,  438. 

Club    Root    of    Cabbage,    339;    de- 
scribed, 340,  340*. 
Coconut  oU,  279. 
Coenocyte,  316,  316*. 
Coffee  tree,   492,   493*;  flowers  and 

berry,  492,  493*. 
Coleochaete,  313,  313*. 
Coleochaete  scutaia,  313*. 
Coleoptile,  63,  102,  63*,  102*. 
Coleorhiza,  63,  63*. 
CoUenchyma,    128*;    described,    129, 

193*. 
Color,  of  flowers,  10;  inheritance  in 

endosperm  of  com,  549*. 
ColumeUa,  416. 
Column,  26. 
Comfrey,  fruit,  86*. 
Common  Barberry,  396. 
Common  Bean,  seedling,  107,  106*. 
Common     Cabbage,     flowers,     28*; 

plants,  172*,  528*. 
Companion    cells,    131,    190*,    193*, 

195*. 
Complete  flowers,  13. 
Composite  family  (Compositae),  492, 

494*. 
Composite  flower,  24. 
Compound  leaf,  237,  237*,  238*. 
Compound  pistil,  15. 
Conceptacle,  323,  322*,  407*,  411*. 
Conditions     affecting     transpiration, 

263. 
Conductive  tissues,    130;   of  leaves, 

244. 
Confervoid  Algae  (Conf ervales) ,  302; 

described,  310. 
Conidia,  356,  394,  357*,  360*,  373*, 

375*. 


INDEX 


571 


Conidiophores,  356,  357*,  359*,  360*, 
373*,  374*,  375*. 

Conidiospores,  356,  357*,  359*,  360*. 

Coniferyl  alcohol,  282. 

Coniferin,  282. 

Conin,  283. 

Conium  mnculalum,  283,  488. 

Conjugation,  302;  in  Bread  Mold, 
361,  363*. 

Conjugating  Algae  (Conjugales),  302; 
described,  314. 

Convolvulaceae,  490. 

Core,  of  apple,  78*. 

Cork,  128,  199,  199*. 

Cork  cambium,  147,  199,  147*,  185*, 
199*. 

Cork  Oak,  476,  477*. 

Corky  bark,  197,  198*. 

Corky  rind,  127,  127*,  128*. 

Corm,  179;  described,  182,  182*. 

Corn  Cockle,  74,  481. 

Com,  life  cycle,  f*;  plant,  16*; 
flowers,  17-20,  17*,  18*,  19*; 
pistil,  34,  36;  stigma,  43*;  devel- 
opment of  seed,  44*;  embryo  sac, 
40,  39*;  effect  on  ear  of  cross- 
pollination,  51;  kernel,  62,  63*, 
64*;  seedling,  102,  103,  101*;  fi- 
brousroots,  138;  growth  of  radical, 
145*,  151*;  section  of  seedling, 
183*;  cross  section  of  stem,  187, 
187*,  188*;  vascular  buncUe,  188, 
189*,  190*;  leaves,  236*;  section 
of  leaf,  267*;  oil,  279;  smut,  395, 
395*. 

Corn  Smut,  395,  395*. 

CoroUa,  11,  10*,  12*. 

Cortex,  132,  145,  146*,  160*,  188*, 
192,  194*,  198*. 

Corymb,  28,  30,  30*,  32*;  compound, 
32*. 

Cotton,  flower,  14*,  33*;  seed,  59, 
59*;  plant,  487,  487*. 

Cotton-seed  oil,  279. 

Cotyledon,  56,  56*,  63*,  101*,  105*, 
106*,  107*,  108*,  109*,  466* 

Cow  Cockle,  74. 

Cow-herb,  481. 


Covered  Smut  of  Barley,  395. 
Crab  Apple,  210. 
Cronartium  ribicola,  402. 
Cross-pollination,  48;  compared  with 

self-pollination,  52,  53. 
Crossing  and  vigor  of  offspring,  564, 

563*,  564*,  565*. 
Crowfoot    family,    481,    482;    flov/er, 

481*,  482*. 
Crown,  of  Red  Clover,  109,  109*. 
Crown  Gall,  349,  349*. 
Cruciferae,  482. 

Cucumber,  section  of  fruit,  79,  78*. 
Cucurbitaceae,  492. 
Culture  of  small  fruits,  164. 
Cup  Fungi  (Pezizales),  364;  described, 

366,  367*. 
Cup  plant,  236*. 
Cup  spores,  398. 
Curled  Dock,  seeds,  75*,  76*. 
Cuticle,  246,  250*. 
Cutin,  245,  276. 
Cuttings,     225,     226*,     228*,     229*, 

230*. 
Cyanophyceae,  297. 
Cycad,  445;  described,  446-451,  446*. 
Cycas  revoluta,  446. 
Cyme,  28,  31*;  scorpoid,  32*;  con/m- 

bose,  32*;  typical,  32*. 
Cypripedium,  499*. 
Cystocarp,  327,  326*,  328*. 
Cytase,  285. 
Cyiisus  Adami,  231. 
Cytoplasm,  114,  327,  326*,  328*. 
Cytology,  2. 

Dakota  Amber  Sargo,  560*. 
Dandelion,  flowers,  24,  24*;  rosette, 

241,  241*. 
Dangers  resulting  from  transpiration, 

265. 
Darwin,  Erasmas,  514;  Charles,  516, 

517*. 
Darwin's  explanation  of   Evolution, 

516. 
Darwinism,  objections  to,  522. 
Date  seeds,  hemi-cellulose  in,  279. 
Datura  Stramonium,  491,  490*. 


572 


INDEX 


Deciduous  trees,  268;  forests,  508, 
509*. 

Deliquescent  stem,  168,  168*. 

Demonstration  of  osmosis,  116,  116*. 

Depth  of  root  systems,  141. 

Desmids,  314,  314*. 

Destructive  Toadstools  and  Bracket 
Fungi,  388. 

Determinants,  536. 

Development,  of  ovule  into  a  seed, 
43,  43*,  44*,  45*;  of  Bean  seed- 
ling, 107,  106*;  of  corn  seedling, 
101*,  103*;  of  Clover  seedling, 
109,  109*;  of  embryo  in  Angio- 
sperms,  466,  466*,  467*;  of  Onion 
seedling,  107,  105*;  of  ovule  and 
pollen  tubes  in  Pine,  455-457, 
455*,  457*;  of  wheat  seedling, 
104*. 

DeVries,  Hugo,  524,  524*. 

Dextrose,  277. 

Diadelphous  stamens,  15,  14*. 

Diagrams  of  inflorescences,  32*. 

Diatoms,  231,  231*,  232*. 

Dichotomous  branching,  429. 

Dicotyledons,  170;  herbaceous  stems, 
192,  192*,  193*,  194*,  195*,  196*; 
woody  stems,  197,  198*,  199*, 
200*,  201*,  202*;  venation  of 
leaves,  245,  471,  244*. 

Diffusion,  of  liquids  and  gases,  95. 

Dionaea  muscipula,  132,  273*. 

Direction  of  growth  in  roots,  factors 
influencing,  150,  151*. 

Dioon,  staminate  strobilus,  447*. 

Discovery  of  the  cell,  112. 

Dissemination  of  seeds  and  fruits,  82; 
by  animals,  84,  85*,  86*;  by 
explosive  or  spring-like  mechan- 
isms, 87,  87*;  by  water,  84;  by 
wind,  82,  83*,  84*. 

Distribution  of  plants,  5. 

Diversity  of  plant  forms,  5. 

Divisions,  in  classification,  291;  of 
Fungi,  252. 

Dock,  59,  60,  128*. 

Doctrine  of  Special  Creation,  514. 

Dodder,  59,  163,  163*,  489. 


Dogbane,  133. 

Doll  rag  germinator,  99,  99*. 

Dominance,  law  of,  543;  impure,  545; 

pure,  545. 
Dominant  characters,  543. 
Dormant  buds,  211. 
Downy    Mildews,    355;    of   Ginseng, 

358,  361*;  of  Grapes,  355,  355*, 

356*;     of    Potatoes,    357,    257*, 

358*,  359*,  360*. 
Double  fertiUzation,  43,  465,  465*. 
Drosera,  132,  272*. 
Dry  plain  societies,  509. 
Duckweeds,  9,  505. 
Dutchman's  Breeches,  170. 

Earthstar,  391,  391*. 

Ecbalium  Elaterium,  87*. 

Ecology,  plant,  2;  nature,  500. 

Ecological  factors,  501. 

Ecological  societies,  504. 

Economic  Botafty,  3. 

Ectocarpus,  320,  321*. 

Edible  Boletas,  387*. 

Elaboration  of  foods  into  plant 
structures,  95. 

Elaters,  411. 

Ehn,  9;  family,  478;  tree,  168*,  176*. 

Embryology,  2. 

Embryo  sac,  39;  of  Corn,  40,  39*,  44*; 
of  oats,  40,  40*,  45*;  of  Red 
Clover,  40,  38*,  42*,  45*;  of  To- 
mato, 43*. 

Embryo,  development  from  fertilized 
egg,  43,  44,  44*,  45*;  of  Apple 
and  Squash,  59*;  parts  of  Bean 
embryo,  56,  56*;  of  Corn,  44*; 
parts  of  corn  embryo,  63,  63*, 
64*;  of  Cotton,  59,  59*;  dicotyle- 
donous type,  466,  466*;  function, 
55;  monocotyledonous  type,  466, 
467*;  of  Oats,  45*;  of  Potato  and 
Buckwheat,  60*;  of  Red  Clover, 
45*;  of  tomato,  43*. 

Endodermis,  of  roots,  147. 

Endogenous  development,  197. 

Endophytes,  297. 

Endolhia  parasitica,  371,  372*. 


INDEX 


573 


Endosperm,  40;  in  Boan  type  of  soeds, 
58,  59*;  of  Buckwheat  and  Flax 
type  of  seeds,  60,  60*;  of  Corn, 
44;  of  Cycads,  449,  449*;  in  Grass 
type  of  seeds,  62;  in  hybrid  Corn, 
51,  51*;  kinds  in  Corn,  63,  64, 
63*;  of  paf,s,  45;  of  Shepherd's 
Purse,  466*;  of  Tomato,  43*;  in- 
heritance, 548*,  549*;  location 
in  seed,  58,  60,  468. 

Endosperm  nucleus,  40,  38*,  39*,  40*. 

Envelopes  of  the  flower,  11. 

Enzymes,  93,  284. 

Epidermis,  as  an  absorptive  structure, 
145,  146*;  cellular  structure, 
246*,  247*,  250*;  as  a  protective 
structure,  127,  127*;  of  leaves, 
245;  modifications  for  protection, 
266,  266*. 

Epigaea,  490. 

Epigynous  flowers,  15,  15*. 

Epiphytes,  163. 

Equisetales,  435-438. 

Equisetum  arvense,  435,  436*,  438; 
palustre,  435;  pratense,  435. 

Erect  stems,  172. 

Ergot,  369,  370,  371*. 

Ericaceae,  489. 

Essential  organs  of  the  flower,  11. 

Etiolation,  220. 

Evolution,  plant,  2,  290;  meaning  and 
theories,  513-5^4;  experimental, 
524-535. 

Euglena  gracilis,  230,  230*. 

Euphorbiaceae,  486,  486*. 

Eusporangiates,  430. 

Excretions  of  roots,  161. 

Excurrent  stem,  168,  168*. 

Exogenous  stems,  197. 

Explosive  mechanisms  of  seeds,  87, 
87*. 

Exposure  of  leaves  to  light,  237. 

Factors,  ecological,  501-504;  in- 
fluencing the  direction  of  growth 
in  roots,  150,  151*;  influencing 
photosynthesis,  257-260;  that 
hinder  absorption  by  roots,  159. 


Families,  in  classification,  291;  Cat- 
tail, 495;  Composite,  492;  Brrch 
and  Oak,  476;  Birch,  475;  Buck- 
wheat, 478;  Buttercup,  481;  Elm, 
478;  Goosefoot,  480;  Gourd,  492; 
Grass,  495;  Heath,  489;  Lily, 
498;  Madder,  492;  Mallow,  487; 
Maple,  487;  Mustard,  482;  Night- 
shade, 491;  Orchid,  494;  Palm, 
497;  Parsley,  488;  Pea,  484; 
Pink,  480;  Rose,  483;  Spurge, 
486;  Sweet  Potato,  490;  Walnut, 
474;  Willow,  473. 

Farm  Crops,  1,  4. 

Fascicled  root  system,  140,  140*. 

Fats,  277. 

Fatty  oils,  279. 

Female  gametophyte,  of  Angio- 
sperms,  463,  464*,  465*;  of 
Cycads,  448,  449*;  of  Equise- 
tum, 438,  438*;  of  Pines,  455, 
455*;  of  Selaginella,  442,  443*. 

Fermentation,  122;  Bacteria  of,  345, 
345*. 

Fern  gametophyte,  431,  432*,  433*. 

Fern  plants,  292;  described,  425-444; 
life  cycle,  433*. 

Fertilization,  41;  described,  42,  42*; 
double,  43;  effect,  50;  in  Algae, 
302;  in  Angiosperms,  464;  in 
Cycads,  450;  in  Pines,  456. 

Fibrous  root  system,  138,  138*. 

Field  Dodder,  seeds,  75*. 

Filament  of  stamen,  41,  41*. 

Fihcales,  426-435. 

Fitness  for  environment,  520. 

Flagellates,  329. 

Flax  family,  486. 

Flower  buds,  207. 

Flowers,  general  characteristics,  9; 
apetalous,  13,  11*;  arrangement, 
26-32;  as  structures  i)eculiar  to 
Angiasperms,  459;  of  Bean 
family,  23,  23*;  of  Beech  family, 
476,  476*;  of  Birch  family,  475, 
475*;  color,  10;  of  Buckwheat 
family,  478*,  479*,  complete,  13, 
10*;  of  Composites,  24,  24*;  ol 


574 


INDEX 


Corn,  17,  16*,  17*,  18*,  19*;  of 
Crowfoot  family,  481,  481*;  of 
Elm  family,  478,  477*,  478*; 
epigynous,  15,  15*;  function,  10; 
gamopetalous,  13,  12*;  of  Goose- 
foot  family,  480,  480*;  of  Grass, 
17;  hypogynous,  16,  15*;  incom- 
plete, 13,  11*;  of  Mallow  family, 
14,  14*;  of  Mustard  family,  482, 
483*;  of  Oats,  20,  20*,  21*;  odor, 
10;  of  Orchids,  26,  25*;  of  Pars- 
ley family,  488,  488*;  parts,  11, 
10*;  of  Pea  family,  484,  485*; 
perigynous,  16,  15*;  of  Pink 
family,  480,  481*;  pistillate,  14; 
polypetalous,  13,  10*;  of  Rose 
family,  483,  484*;  size,  9;  some 
particular  forms,  16;  of  Spurge 
family,  486,  486*;  staminate,  13; 
unisexual,  13,  12*,  16*,  17*,  18*, 
19*;  of  Walnut  family,  474,  474*; 
of  Wheat,  22,  22*;  of  Willow 
family,  473,  473*. 

Flowering  glume,  18,  18*. 

Flowering  plants,  6,  289;  life  cycle,  6, 
7*,  469*. 

Fluctuating  variations,  525. 

Foods,  elaboration  into  plant  struc- 
tures, 95;  manufacture  by  leaves, 
252;  nature  of,  120;  reserve,  277; 
use,  273. 

Foot  of  sporophyte,  410,  410*. 

Forestry,  1,  4. 

Formation  of  buds,  208. 

Foxtail,  seeds,  76*. 

Framework  of  plants,  275. 

Free-swimming  societies,  505. 

Free  water  in  the  soil,  153. 

Fronds,  428,  427*. 

Fructose,  277. 

Fruits  of  Flowering  plants,  nature 
and  types,  77;  aggregate,  80; 
berry  type,  77,  77*;  blackberry 
type,  79,  79*;  definition,  81; 
dissemination,  82-88;  Pineapple 
type,  80,  80*;  pepo  type,  79,  78*; 
pome  type,  78,  78*;  multiple,  80; 
some  other  types,  80,  81*,  82*; 


stone  type,   77*,  78;  strawberry 

type,  79,  79*. 
Fruit  sugar,  277. 
Fucales,  321-324. 
Fucoxanthin,  318. 
Function,  meaning,  6;  of  cells,  112;  of 

flowers,  10;  of  leaves,  233,  252; 

ot  roots,  136,  137;  of  seeds,  55, 

56;  of  stems,  168. 
Fungi,  289,  336;  described,  351-404; 

Alga-like,  353-363;  basidia,  382- 

403;    divisions,    352;    Imperfect, 

404; sac,  363-379. 
Fungi  Imperfecti,  352. 
Funiculus,  38,  38*. 

Galium  Aparine,  240*. 

Galton,  Francis,  538. 

Gametes,  302;  combinations  and  the 
Mendelian  ratio,  556;  kinds,  302; 
segregation  and  purity,  543. 

Gametophyte  generation,  411;  in 
Angiosperms,  463,  464,  462*, 
464*;  in  Cycads,  448,  450,  449*; 
in  Equisetum,  437,  438*;  in 
Ferns,  431,  432*,  433*;  in  Liver- 
worts, 411,  412*;  in  Lycopodium, 
440;  in  Moss,  417,  418*,  421*, 
423*;  in  Pines,  455,  456,  455*;  in 
Selaginella,  442,  443*. 

Gamopetalous  flowers,  13,  12*. 

Gamosepalous  flowers,  13. 

Garden  Pea,  flower  and  pod,  35*;  in- 
heritance in,  540,  541,  542*. 

Gasteromyces,  384;  described,  389. 

Geaster,  391,  391*. 

Gemmae,  409. 

Gemmae  cups,  409,  407*. 

Genera,  in  classification,  291. 

Genes,  536;  active  and  latent,  537. 

Genetics,  538. 

Genotype,  554. 

Geotropism,  150,  151*. 

Geranium,  bending  toward  light, 
243*. 

Germination  of  seeds,  89-98;  effect 
of  temperature  on  rate,  90; 
moisture  requirement,  91;  neces- 


INDEX 


575 


sary  conditions,  S9;  oxyfjcMi  re- 
quirement, 92;  processes,  93-9S; 
temperature  requirement,  89. 

Germinators,  99,  99*,  100*. 

Germ-plasm,  532. 

Giant  Evening  Primrose,  530,  530*. 

Gill  Fungi,  385. 

Gleba,  390. 

Gleooapsa,  298. 

Gliadins,  281. 

Globulins,  281. 

GloincreUa  rufomacidans,  379,  379*. 

Glucose,  277. 

Glucosides,  281. 

Glumes,  17,  18*;  empty  or  outer,  18, 
18*;  flowering,  18. 

Glutelins,  281. 

Glutenin,  281. 

Grafting,  225;  described,  227,  232*. 

Gramineae,  495. 

Grape  vine,  176,  176*. 

Grape  Downy  Mildew,  355, 355*,  356*, 
357*. 

Grape  sugar,  277. 

Grass,  flowers,  17-23,  17*,  18*,  19*, 
20*,  21*,  22*;  family,  495;  seed- 
Kngs,  102,  102*,  103*,  104*. 

Grass  type  of  seeds,  61-67. 

Green  Algae,  301-318;  confervoid, 
310-314;  conjugating,  314-316; 
in  relation  to  Lichens,  380;  tubu- 
lar, 316-318;  unicellular  motile, 
302-307;  unicellular  non-motile, 
307-310. 

Green  Foxtail,  seed,  76*. 

Green  Molds,  364;  described,  375, 
375*. 

Grizzly  Giant,  174*. 

Growth  in  roots,  factors  influencing 
the  direction,  150,  151*. 

Growth  of  Stems,  213-221;  char- 
acter and  rate,  214;  factors  in- 
fluencing, 217-221;  grand  period, 
216;  phases,  213;  primary  and 
secondary,  214;  regions,  213, 
214*. 

Guard  ceUs,  247,  246*,  247^ 

Gulfweeds,  319;  described,  323. 


(iymnosperms,  67,  289;  described, 
445-458;  life  cycle,  457*;  seed 
and  seedling,  67,  67*. 

Gymnosporangium,  401,  400*,  401*, 
402*,  403*. 

Halberd-shaped  leaves,  479*. 

Hard  seeds,  69. 

Hardwood  cuttings,  227,  230*. 

Harmogonia,  299,  299*. 

Haustoria,  352,  356*. 

Head,  30,  29*,  32*. 

Heart  Rot,  White  and  Red,  389. 

Heart  wood,  202,  200*. 

Heath  family,  489. 

Heathers,  490. 

Heliotropism,  152. 

Helvellales,  364. 

Hemi-celluloses,  278. 

Hemlock,  451. 

Hepaticae,  405. 

Herbaceous  stems,  171;  structure, 
192-197. 

Herbarium  Mold,  375. 

Heredity,  535-556;  experimental 
study,  537-556;  importance  of 
its  study,  537;  nature,  535;  physi- 
cal basis,  535. 

Heterocysts,  299,  299*. 

Heterogametes,  302. 

Heterogamous  sexuality,  304. 

Heterospory,  442. 

Heterozygous,  544. 

Hibiscus  Trionum,  487. 

Hickory,  pistillate  flower,  81*;  species 
and  economic  importance,  475; 
tap-root,  139,  139*. 

Hilum,  of  seed,  57,  57*;  of  starch 
grain,  256*. 

Histology,  2. 

Holdfasts,  135. 

Homosjoory,  442. 

Homozygous,  544. 

Hooke,  Robert,  113. 

Horny  endosperm,  64,  63*,  64*. 

Horse  Nettle,  492,  491*. 

Horsetails,  426;  described,  435. 

Horticulture,  1. 


576 


INDEX 


Host,  337. 

Huckleberries,  489. 

Hugo  De  Vries,  524,  524*. 

Hugo  Von  Mohl,  113. 

Humus,  154. 

Hybridization,  5G1. 

Hydrocyanic  acid,  282. 

Hydrodictyon,  309. 

Hydrodictyon  reticulatum,  309*. 

Hydrophytes,  504. 

Hydnum,  388,  387*. 

Hydrophytic  societies,  504. 

Hydrotropism,  151,  151*. 

Hygroscopic  water,  153. 

Hymenium,  365. 

Hymenomycetes,  384. 

Hypha,  352;  infection,  394. 

Hypocotyl,  of  Bean,  56,  56*,   106*; 

of  Bean  type  of  seedlings,  107; 

of  Corn,  63;  of  Onion,  105*;  of 

Red  Clover,  109*;  of  Squash,  107, 

107*. 
Hypogynous  flowers,  16,  15*,  484*.    . 

Impatiens,  88. 

Imperfect  Fungi,  352;  described,  404. 
Inbreeding,  results,  564,  563*,  564*. 
Incomplete  flowers,  13,  11*,  12*. 
Indeterminate  inflorescence,  28,  28*. 
Indian  Turnip,  18,  18*. 
Indusium,  429,  430*. 
Infection  hypha,  394. 
Inferior  ovary,  15,  15*. 
Inflorescence,    26;    determinate,    28, 

32*;  indeterminate,  28,  32*. 
Inheritance,  517;  described,  519;  in 

ears  of  Corn,  551*;  in  heads  of 

Wheat,  552*. 
Inorganic  evolution,  513. 
Integument,  38,  35*,  36*,  38*,  39*;  of 

Cycads,  448,  449*;  of  Pines,  454, 

455*. 
Interdependence  of  shoot  and  root, 

136. 
Internodes,    166;    of    Corn    seedling, 

183*. 
Interrupted  Fern,  431,  431*. 
Invertase,  285. 


Investigations  since  Mendel,  547. 

Involucre,  30,  29*. 

Iodine  test  for  starch,  256. 

Irish  Moss,  326,  325*. 

Irish  Potato,  roots,  142;  eyes  and  scale 

leaves,    181,    179*;   i)ropagation, 

226,  226*. 
Iron,    a   mineral   element    for   crops, 

157. 
Isoetes,  438. 
Isogametes,  302. 

Jerusalem  Artichoke,  181. 
Jimson  Weed,  61. 
Johnson  Grass,  177. 
Juglandaceae,  474. 
Jungcrmaniales,  407. 

Kafir  Corn,  142. 

Keel,  24,  23*. 

Kelps,  319,  319*,  320*. 

Kernel  of  Corn,  44*;  structure,  62,  63, 

64*. 
Kernel,  of  Oats,  22,  45*;  of  Wheats  65, 

65*. 

Lactiferous  vessels,  133. 

Lady's  Thumb,  seeds,  75. 

Lamarck's  Evening  Primrose,  529. 

Lamarck,  515;  his  explanation  of  evo- 
lution, 515. 

Laminar ias,  319. 

Lamb's  Quarter,  seeds,  75*,  76*. 

Larches,  451. 

Large  seeded  Alfalfa  Dodder,  76*. 

Lateral  buds,  206,  206*. 

Lateral  flowers,  27. 

Law  of  Dominance,  543. 

Layering,  225,  231*. 

Leader,  312. 

Leaf  Blight  of  Cotton,  404. 

Leaf  buds,  207. 

Leaflet,  237,  237*,  238*. 

Leaf  traces,  185,  186*. 

Leaves,  233-286;  auricles,  235,  235*; 
base,  235;  blade,  235;  cellular 
structure,  246-252;  compound, 
237,    237*;    development,    234: 


INDEX 


577 


general  structure,  242-246;  man- 
ufacture of  food,  252;  margin, 
235;  mosaic,  241,  241*;  parts, 
234;  perfoliate,  235,  23G*;  pri- 
mary and  secondary,  234;  of 
Corn,  235,  236*;  rosette,  241, 
241*;  sessile,  235,  235*;  sheath, 
235;  simple,  237,  237*;  special 
forms,  270;  stipules,  234,  235; 
transpiration,  260;  use  of  the 
photosynthetic  food,  273. 

Legume,  80,  485,  82*,  485*. 

Legumelin,  281. 

Legumes,  156. 

Legimiinosae,  58;  described,  484. 

Legumin,  281. 

Lemma,  17,  22,  18*,  19*,  22*. 

Lemons,  77,  133. 

Lenticels,  184,  184*,  185*. 

Leptosporangiate,  430. 

Lettuce,  prickly,  85;  Sea,  310,  310*; 
Wild,  495. 

Leucin,  281. 

Leucoplast,  115. 

Leucosin,  281. 

Levulose,  277. 

Lichens,  363;  described,  379-382. 

Life  Cycle,  of  Angiosperms,  469*;  of 
Ferns,  433*;  of  Flowering  Plants, 
6,  7*;  of  Marchantia,  412*;  of 
Moss,  421*;  of  Pine,  457*;  of 
Wlieat  RiLst,  399*;  of  Cedar 
Rust,  403*. 

Light,  as  influencing  growth,  220;  as 
an  ecological  factor,  502;  as 
related  to  leaves,  237;  as  related 
to  photosynthesis,  257,  257*;  as 
relat(Hl  to  transpiration,  263. 

Lignin,  115,  276. 

Ligulate  flowers,  253,  24*. 

Ligule  or  rain  guard,  235. 

Lilac  Mildew,  373. 

Lily,  embryo  sac,  465*;  family,  498. 

Linaceac,  486. 

Linseed  oil,  279. 

Lipase,  94. 

Liverworts,  504;  described,  406-417. 

Loam,  154. 


Locules,  of  anther,  41,  41*;  of  fruit, 
77,  78,  77*,  78*;  of  ovary,  34,  38*. 

Lodiculcs,  17,  20,  18*,  19*,  22*. 

Longevity  of  seeds,  67;  discussed,  71- 
74. 

Macrocystis,  319,  319*. 

Madder  family,  492. 

Magnesium,  157. 

Main  roots,  136. 

Malic  acid,  285. 

Mallow  family,  487. 

Maltase,  285. 

Maltose,  277. 

Manufacture  of  food,  in  leaves,  252- 
260;  in  stems,  169. 

Maple,  genus,  292;  family,  487. 

Marchantias,  407-415. 

Marchantiales,  407-415. 

Marchantia  polymorpha,  407-414. 

Margin,  of  leaves,  235. 

Marguerite,  494,  494*. 

Mass  culture,  559. 

May  Apple,  or  Mandrake,  180,  ISO*. 

Meadows,  508. 

Medicago  sativa,  fruits,  82*. 

Medullary  rays,  132,  197,  198*,  201*, 
202*. 

Megaspore,  442,  441*;  formation,  463, 
462*;  mother  cells,  463,  461*, 
462*. 

Megasporangimn,  442,  441*. 

Megasporophyll,  442,  441*. 

Megastrobilus,  448*. 

Melon  type  of  fruit,  79,  78*. 

Membrane,  of  the  cell,  114,  115*;  of 
the  nucleus,  114,  114*;  .senu- 
permeable,  117. 

Mendel,  Gregor,  539,  539*;  lUs- 
coveries,  541,  542*;  experiments, 
540;  law  of  dominance,  543. 

Mendel's  law,  546;  use  in  plant  bre(;d- 
ing,  562;  value,  547. 

Mendehan  ratio  and  the  combination 
of  gametes,  556. 

Mendelism,  :is  demon.strated  in  Corn, 
548*,  549*,  ,550*,  551*;  ;is  demon- 
strated in  Carden  Pea,  541-546, 


578 


INDEX 


542*;  as  demonstrated  in  Wheat, 
552*. 

Meristom,  126,  204,  126*,  204*. 

Meristematic  tissue,  126,  126*;  of 
buds,  204,  204*;  of  Grass  stems, 
213,  214*;  of  root  tip,  143,  144*. 

Mesophyll,  244;  described,  246,  250*. 

Mesophytic  societies,  504;  described, 
506. 

Metabolism,  116,  273;  anabolic  and 
catabolic,  273. 

Micro-organisms  of  the  soil,  155. 

Micropyle,  39,  57,  38*,  57*. 

Microsphaera,  373. 

Microspore,  442,  441*;  formation, 
461,  462*;  mother  cells,  461,  461*. 

Microsporangimn,  442,  441*. 

MicrosporophyU,  442,  441*. 

MicrostrobihLs,  446,  447*. 

Middlings,  65. 

Midrib,  245. 

Mildews,  Downy,  355-360;  Powdery, 
373-375. 

Milkweed,  83,  133,  83*. 

Mistletoe,  163. 

Mixed  buds,  207. 

Moisture,  given  ofT  in  transpiration, 
260,  260*;  of  the  soU,  217;  re- 
quired for  germination,  91. 

Molds,  155;  Blue  and  Green,  375-377; 
True,  360-363;  Water,  353-360. 

Monadelphous  stamens,  15. 

Monocotyledons,  embryo,  466,  467*; 
families,  495;  origin,  471;  seeds, 
66;  seedling,  102*,  103*,  104*, 
105*;  stems,  170;  structure  of 
stems,  187-192. 

Monoecious,  14. 

Moonworts,  435,  434*,  435*. 

MorcJiella  esculenta,  365,  365*,  366*. 

Morels,  364,  365*. 

Morning  Glory,  cotyledons.  111; 
plumule,  109;  stem,  168,  175, 
177*. 

Morphine,  283. 

Morphology,  2,  289. 

Mosaic,  of  leaves,  241,  241*. 

Mosses,  292,  405;  described,  417-424. 


Mucorales,  360-363. 

Mullein,  leaf,  266*. 

Multiple  fruits,  80,  80*. 

Multiplication  of  cells,  123,  125*. 

Musci,  405. 

Muslu-ooms,  352,  382,  384,  385*,  386*. 

Mustard,   Black,   483,   483*;  family, 

482. 
Mutant,  627. 
Mutation,    525,    528*;    in    Evening 

Primrose,  529. 
Mutation  theory,  527;  compared  with 

Darwinism,  531. 
MyceUum,  352. 
Mycorhiza,  155,  155*. 
Myrsiphyllum,  169. 
Myxobacteria,  350. 
Myxomycetes,  336;  described,  336- 

341;  economic  importance,  339. 

Naked  Ascus  Fungi,  364,  377,  376*. 

Naked  buds,  205. 

Nasturtiums,  241*. 

Natiu-al  history,  524. 

Natural  selection,  517. 

Navicula  viridis,  332*. 

Nectar,  133. 

Nectar  glands,  133,  133*. 

Nectarine,  528,  561. 

NemaUon,  326,  326*. 

Nereocystis,  319. 

Nerves,  245. 

Net-veined  leaves,  245,  244*. 

Nicotine,  283. 

Nidularia,  392,  392*. 

Nightshade  family,  491. 

Nitrogen,  133. 

Nodes,  166. 

Nostoc,  209,  208*. 

NuceUus,  38,  448,  454,  35*,  36*,  38*, 
449*,  455*. 

Nuclear  membrane,  114,  114*. 

Nuclear  sap,  114,  114*. 

Nucleolus,  114,  114*. 

Nucleo-proteins,  281. 

Nucleus,  39;  described,  114,  114*; 
endosperm,  40,  44,  51;  of  leaf 
cell,     251,     251*;    primary    en- 


INDEX 


579 


dosperm  nucleus,  40,  38*,   39*; 

tube,  42,  42*. 
Nut  type  of  fruit,  80,  81*. 
Nux  vomica,  283. 

Oak,  family,  476;  flowers,  476,  470*; 

plain  sawed,  203*;  quarter  sawed, 

203*;  uses,  476,  477*. 
Oat,  flower,  20,  20*,  21*;  kernel,  66; 

Smut,  393. 
Objections  to  Darwinism,  522. 
Oedogonium,  312,  312*. 
Oenothera,    brevistylis,    530;    gigas, 

530,    530*;   laevifolia,   530;   La- 

marckiana,  529,  529*. 
Oils,  273;  fatty,  279;  volatile,  281. 
Onion,  section  of  root,  144*;  section 

of  bulb,  181*;  seedling,  107,  105*. 
Onoclea  sensibilis,  431,  431*. 
Ontogeny,  291. 
Oogonium,  312,  312*. 
Oospore,  302. 
Open  bundles,  195. 
Opening  of  buds,  205,  205*. 
Optimum  temperature  for  germina- 
tion, 90. 
Orchid,  family,  499;  flowers,  26,  25*, 

499*. 
Order,  in  classification,  291. 
Organ,  6;  essential  organs  of  flowers, 

11;  sex,  302. 
Organism,  6. 
Organic  evolution,  513. 
Origin  oj  Species,  517. 
Orthotropous  ovule,  463, 
Oryzenin,  281. 
Oscillatoria,  298,  298*. 
Osmosis,  94;  as  related  to  cell  activity, 

117;  described,  116,  116*. 
Osmotic  pressure,  119. 
Osmunda  Claytonia,  431,  431*. 
Ovary,  33,  33*,  34*;  inferior,  15,  15*; 

structure,     34,    34*,     35*,     36*; 

superior,  16,  15*. 
Ovules,  34,  34*,  35*,  36*;  of  Angio- 

sperms,    463;    of    Cycads,    448, 

448*,  449*;  cellular  structure,  39, 

39*;  development  to  seeds,  43, 


43*,  44*,  45*;  of  Pines,  454,  454*, 

455*;  parts,  38,  38*;  relation  to 

seed.s,  37. 
Ovulate   strobilus,    of    Cycads,    446, 

447,  448*;  of  Pines,  453,  454*. 
Oxalic  acid,  285. 
Ox-eye  Daisy,  86. 
Oxygen,     for    germination,     92;    in 

respiration,  121,  269. 

Palea,  17,  18*,  19*. 
Paleobotany,  3. 
Palisade  tissue,  249,  250*. 
Palmaceae,  497, 
Palm  family,  497. 
Pandorina,  304,  305*. 
Panicle,  23,  32;  of  Oats,  120*. 
Papain,  284. 
Papaw,  284. 
Pappus,  26,  24*. 

Parallel-veined  leaves,  245,  244*. 
Paraphysis,  365,  366*. 
Parasite,  337. 
Parasitic  roots,  163,  163*. 
Parenchyma  cells,  127. 
Parsley  family,  488. 
Parthenocarpic  fruits,  50. 
Parthenogenesis,  355,  467. 
Parthenogenetic  fruits,  50, 
Parthenocarpy,  467. 
Parts,  of  a  flower,  11,  11*;  of  a  leaf, 
234;  of  a  pistil,  33;  of  a  plant,  6, 
Pathogenic  Bacteria,  346,  347*. 
Pea  famUy,  484-486. 
Pea  type  of  seedling,  109,  110*, 
Pear  Blight,  348,  348*, 
Peat,  154. 
Peat  Moss,  422. 
Pectic  substances,  276. 
Pediastrum,  309,  308*. 
Pediastrum  boryanum,  308*. 
Pedicels,  20,  20*. 
Pedigree  cultm-e,  560. 
Penicillium,  375;  described,  376,  376*. 
Pepo  type  of  fruit,  79,  78*. 
Pepsin,  94. 
Peptases,  284. 
Peptones,  284. 


580 


INDEX 


Perennial  stems,  171. 

Perfect  flower,  13,  11*. 

Perianth,  11,  460,  11*,  460*. 

Pericarp,  61,  66*. 

Pericentral  cell,.  328. 

Pericycle,  147. 

Peridium,  390. 

Perigynous  flower,  16,  15*. 

Perisperm,  46. 

Perisporiales,    364;    described,    373- 

375. 
Peristome,  421. 
Perithecium,  369,  370*. 
Permanent  root,  103. 
Peronosporales,  353;  described,  355- 

360. 
Petal,  12,  11*. 
Petiole,  234. 
Peziza,  366,  367*. 
Pezizales,  364;  described,  366-369. 
Phaeophyceae,  318-324. 
Phallus  impudicus,  391,  392*. 
Pheflogen,  185*. 
Phenotype,  554. 
Phloem,  130,  146,  188. 
Phosphoras,  as  a  soil  constituent,  157. 
Photosynthesis,  169;  described,  252- 

260;  factors  influencing,  257. 
Phj'cocyanin,  297,  324. 
Phycoerythrin,  324. 
Phycomycetes,  352;  described,  353- 

363. 
Phylogenetic  divisions,  292. 
Phylogeny,  291. 
Phylloglossum,  438. 
Physical  basis  of  heredity,  535. 
Phytophthora  cactorum,  358,  361*. 
Phylophlhora    infestans,    357,     358*, 

359*,  360*. 
Pigments,  281;  described,  283. 
Pileus,  386,  386*. 
Pilobolus,  362. 

Pineapple  type  of  fruit,  80,  80*. 
Pine  Blister-Rust,  402. 
Pines   (Pinaceae),   451-458;  gameto- 

phytes,  455-457;  life  cycle,  457*; 

seed,  457;  sporophyte,  452,  451*; 

strobUi,  452-455. 


Pink,  family,   480;   flower,  481,  15*, 

481*. 
Pistil,    11,    11*;    compound,    15;    de- 
scribed,    14,     462;    simple,    15; 
structure,  33-37. 
Pistillate  flower,  14,  12*;  of  Corn,  18, 

19*. 
Pitcher  Plant,  132,  272*. 
Pith,  187,  187*. 
Pitted  vessels,  188,  190*. 
Placenta,  77,  34*. 
Plagiotropism,  173. 
Plant  Breeding,  557-565. 
Plant  Ecology,  2. 
Plant  food,  nature,  120. 
Plant  Geography,  3. 
Plant  Pathology,  2. 
Plant  Physiology,  2. 
Plant  succession,  510-512. 
Plasmopara  Viticola,  355,  355*,  356*. 

357*. 
Plastids,  115,  251. 

Plectascales,  364;  described,  375-377. 
Pleurococcus  vulgaris,  294,  307,  294*, 

307*. 
Plowrightia  morbosa,  369,  370*. 
Plum  type  of  fruit,  78,  77*. 
Plumule,  56,  466,  56*,  466*. 
Podophyllum,  180,  180*. 
Poisonous  Toadstool,  384*. 
Pollen,  41,  41*,  42*;  function,  42;  in 

relation  to  external  factors,  49; 

structure,  41,  42*. 
Pollen  chamber,  448,  449*. 
Pollination,  46;  agents,  46;  kinds,  47; 

kinds    giving    best    results,    52; 

nature,    46;    results,    50;    self-, 

close-,  and  cross-,  48. 
Polyadelphous  flowers,  15. 
Polycotyledons,  67. 
Polyembryony,  467. 
Polygonaceae,  478-480. 
Polygonum  Muhlenhergii,  227,  479*. 
PolyiJetalae,  481-489. 
Polypetalous  flowers,  13. 
Polyporaceae,  387.  * 

Polysiphonia,  327,  328*. 
Polysiphonia  violacea,  328*. 


INDEX 


581 


Pome  type  of  fniit,  78,  78*. 

Pond  Lily  society,  511*. 

Pondweed  societies,  505. 

Porella,  415. 

Postelsia,  319. 

Potassium,  as  a  soil  constituent,  157. 

Potato  Blight,  357,  358*,  359*,  300*. 

Potato  scab,  348,  247*. 

Powdery  Mildews,  373-375;  on  Apple 

leaf,  372;  on  Hop,  373*. 
Powdery  Scab  of  Irish  Potato,  340, 

341*. 
Prairies,  508. 

Pressure  within  cells,  118. 
Primary  endosperm  nucleus,  40. 
Primary  growth,  214. 
Primary  leaves,  234. 
Primary  meristems,  127. 
Primary  rays,  203,  201*. 
Primary  root,  102. 
Primary  veins,  245. 
Procarp,  326,  326*. 
Processes  in  cell  activity,  116. 
Promycelium,  394. 
Prop  roots,  139. 
Propagation,  by  roots,   163-165;  by 

stems,  225-233. 
Prostrate  stems,  172,  173. 
Proteases,  284. 
Protective  tissues,  127. 
Protection  against  transpiration,  266. 
Proteins,  256,  280;  kinds,  281. 
Prothallial  cell,  456. 
ProthaUus,  432,  432*. 
Protoascales  (Yeasts),  377,  377*. 
Protococcales,  302;  described,  307. 
Protococcus,  307. 
Protodiscales,    364;    described,    377, 

376.* 
Protonema,  421. 
Protoplasm,  39,  274;  described,  113, 

114*. 
Protoplast,  113. 
Protozoa,  155. 
Pruning,  221-225. 
Pseudopodium,  423. 
Pteridophytes,   292;  described,   425- 

444. 


Pteridosperms,  445. 
Ptomaines,  283. 
Puccinia  Asparagi,  403. 
Pucdnia  graminis,  396-401. 
Puffballs,  382;  described,  389-392. 
Pulsating    vacuoles,    303,    303*;    in 

Euglena,  330,  330*. 
Pumpkin,  flowers,  12,  12*;  length  of 

root  system,  135. 
Purity  and  analysis  of  seeds,  74. 
Pyrenoid,  303,  303*. 
Pyrenomycetales,      364;      described, 

369-373. 
Pyronema,  367,  367*. 

Quack  Grass,  86. 
Quarter-sawed  Oak,  203,  202*. 
Quetelet's  law,  525,  526*. 
Quince,  142. 

Raceme,  28,  28*,  32*. 

Rachilla,  20,  21*. 

Rachis,  20,  20*,  21*. 

Radicle,  of  Bean,  56,  56*;  of  Corn,  63, 
102,  63*,  101*;  development, 
466,  467,  466*,  467*. 

Ranunculaceae,  481 ;  flower,  481*. 

Raphe,  57,  57*. 

Rays,  primary  and  secondary,  203. 

Receptacle,  12,  10*,  11*. 

Recessive  character,  543. 

Red  Algae,  296;  described,  324 '329. 

Red  Clover,  flower,  23,  12*,  23*;  fer- 
tilization in,  42*;  hard  seeds,  69; 
impmities  in  seetls  of,  75*;  Life 
cycle,  469*;  ovule,  38*;  pistil,  36, 
35*;  pollination,  47*;  seedling, 
109,  109*. 

Red  Heart  Rot,  389. 

Reduction  tlivision,  412,  413*;  as  re- 
lated to  segregation,  554,  555*. 

Regions  of  growth,  in  roots,  145,  143, 
145*,  149*;  in  stems,  213,  214*, 
215*. 

Reproductive  tissues,  133. 

Reserve  cellulose,  278,  279*. 

Reserve  foods,  277. 


582 


INDEX 


Resin  ducts,  133. 

Respiration,  96-98,  121-123;  anaero- 
bic, 122;  as  observed  in  leaves, 
269. 

Resting  buds,  204. 

Resting  cell,  299. 

Resting  period  of  seeds,  67-69. 

Results  of  pollination,  50,  51*. 

Reticulated  vessel,  190*. 

Rhizoids,  135;  of  Liverworts,  408, 
407*,  408*;  of  Moss,  418*. 

Rliizome,  179,  180*;  of  Ferns,  427, 
427*. 

Rhizopus  nigricans,  360-363. 

Rhodophyceae,  324-329. 

Riccia,  414,  414*. 

Rind,  of  Corn  stem,  187,  187*;  of 
Potato,  128,  128*. 

Ringing,  211. 

Rivularia,  300,  300*. 

Robert  Hooke,  113. 

Rockweeds,  322-323. 

Root  cap,  63,  143,  63*,  144*. 

Root  hairs,  131,  145,  131*,  146*, 
153*. 

Root  Rot,  388. 

Roots,  primary,  secondary,  permanent 
and  temporary,  102,  103,  101*, 
102*,  103*,  104;  described,  135- 
152;  absorption,  158-161;  adven- 
titious, 140;  depth  and  spread, 
141 ;  direction  of  growth,  150-152; 
excretions,  161;  fascicled,  140, 
140*;  fibrous,  138,  138*;  in  prop- 
agation, 163-165;  in  relation  to 
the  soil,  152-158;  main,  136;  of 
Ferns,  427,  428,  42*7*;  pressure, 
161;  prop  or  brace,  139;  relation 
to  shoot,  136;  tap-root,  139,  139*; 
texture,  136;  types  of  root  sys- 
tems, 138-141;  water,  air,  and 
parasitic,  162. 

Rootstock,  179,  180*;  of  Ferns,  427, 
427*. 

Rosaceae,  483. 

Rose,  family,  483;  flowers,  483,  484*. 

Rosette,  241,  241*. 

Rubiaceae,  492. 


Rutnex  acetosella,  479*. 

Rumex,  coUenchyma  cells  in,  128. 

Runners,  175,  175*. 

Russian  Thistle,  480,  480*. 

Rusts,     396-403;     Apple,     401;     of 

Asparagus,  403;  Black,  396;  of 

Pines,  402. 

Saccharorayces,  377,  377*. 

Saccharose  or  sucrose,  277. 

Sac  Fungi,  352;  described,  363-379. 

Sago  Palm,  446. 

Saponaria  Vaccaria,  74. 

Saponins,  283. 

Saprolegnia,  353,  353*. 

Saprolegniales,  353;  described,  353- 
355. 

Saprophytes,  337. 

Sap  wood,  202. 

Sargasso  Sea,  323. 

Sargassum,  323,  323*. 

Scenedesmus,  308,  308*. 

Sclerenchyma  fibers,  194. 

Sclerotinia  fructigena,  368,  368*. 

Sclerotium,  371,  371*. 

Scouring  Rushes,  435. 

Scutellum,  63. 

Sea  Lettuce,  310,  310*. 

Sea  Palm,  319. 

Seaweeds,  296. 

Secondary  growth,  214. 

Secondary  leaves,  234. 

Secondary  root,  102;  origin,  150, 
150*. 

Secretions  of  plants,  281. 

Secretory  tissue,  133,  133*. 

Seed  coat,  55;  function,  56. 

Seed  plants,  3;  described,  445-470. 

Seedlings,  101;  Common  Bean  type, 
107,  106*;  comparative  size,  110; 
Grass  type,  102,  101*,  102*,  103*, 
104*;  Onion  type,  106,  105*;  Pea 
type,  109,  110*;  types,  102-110. 

Seeds,  dissemination,  82-88;  germi- 
nation, 89-98;  nature,  55;  of 
Cycads,  450;  of  Pines,  457;  purity 
and  analysis,  74-77;  structure, 
56,  57,  56*,  57*;  testing  germi' 


INDEX 


583 


native  capacity,   98-101;  types, 

58-67. 
Segregation  and  i)urity  of  gametes, 

543. 
Segregation  and  reduction   division, 

554. 
Selaginella,  438;  described,   440-444. 
Selection,  557. 
Selection  of  mutants,  561. 
Selective  absorption,  159. 
Self-pollination,  48. 
Semi-permeable  membrane,  117. 
Sepals,  11,  10*,  11*. 
Sessile  leaf,  235,  235*. 
Seta,  420. 
Sex  cells,  302. 
Sex  organs,  302. 
Sheep  Sorrel,  479*. 
Sieve  tubes,  131. 
Silphium  perfoliatum,  236*. 
Simple  stems,  167. 
Sinigrin,  283. 
Siphonales,  302;  described,  316,  316*, 

317*. 
Smartweed,  479*. 
Smuts,  384;  described,  392-396. 
Soil,  as  the  home  of  roots,  152-162; 

as    an    ecological    factor,     502; 

microorganisms,  155;  origin,  152; 

rock  constituents,  152;  solution, 

157;  water,  air,  and  humus,  153. 
Solonaceae,  491. 
Solanin,  283. 

Solanum  Carolinense,  492. 
Solanum  nigrum,  491. 
Sulnnum  rostratum,  492. 
Solitary  flowers,  27. 
Solsola  Kali,  var.  tenuifolia,  480*. 
Somatoplasm,  532. 
Soredia,  381. 
Sorus,  429,  430*. 
Sour  Cherry,  212,  212*. 
Spawn  of  the  Mushroom,  387. 
Special  fomis  of  leaves,  270-273. 
Species,  in  classification,  291. 
Spermatophytes,  292;  described,  445- 

470. 
Sperms,  302. 


Sphagnales,  417;  described.  422-424. 

Sphagnum,  422,  423*. 

Spike,  17,  29,  29*,  32*. 

Spikelets,  of  Corn,  17,  18*,  19*;  of 
Oats,  20,  20*,  21*;  of  Wheat,  22, 
22*. 

Spindle  fibers,  124. 

Spiral  vessel,  188,  189*,  190*. 

Spirillum,  341. 

Spirogyra,  315,  315*. 

Spongospora  subterranea,  341. 

Spongy  tissue,  249,  250*. 

Sporangia,  320,  338. 

Sporangiophore,  437. 

Spore,  338;  of  Bacteria,  343,  343*. 

Sporidia,  394. 

Sporophore,  383. 

Sporophylls,  437,  439,  441,  447,  448, 
452,  453,  454,  460,  462. 

Sporophyte,  of  Angiosperms,  459- 
463;  of  Cycads,  446,  446*;  of 
Equisetales,  436-437;  of  Ferns, 
426-431;  of  Liverworts,  411, 
410*;  of  Lycopodium,  439-440; 
of  Moss,  418,  418*;  of  Pines,  452- 
455;  of  Selaginella,  441,  440*. 

Spurge  family,  486. 

Squash,  arrangement  of  flowers,  27*. 

Squirting  Cucumber,  88,  87*. 

Stamens,  11;  described,  41,  41*. 

Staminate  flower,  13;  of  Corn,  17,  16*, 
17*;  of  Pumpkin,  12*. 

Staminate  strobilus,  446,  447,  447*; 
of  Pines,  452,  452*,  453*. 

Standard,  23,  23*. 

Starch,  as  a  storage  product,  278; 
formation,  255;  test  for,  256. 

Starch  grains,  256,  256*;  structure, 
278,  278*. 

Starch  sheath,  147,  192,  193*. 

Starchy  endosperm,  64. 

Stemonitis,  337*. 

Stems,  aerial,  172;  branching,  167; 
characteristic  features,  166; 
classes,  170;  climbing,  175;  erect, 
173;  functions,  168;  growth,  213- 
221;  of  Ferns,  427;  of  herbaceous 
Dicotyledons,  192-197;  of  Mono- 


584 


INDEX 


cotyledons,  187-192;  of  woody 
Dicotyledons,  197-203;  pros- 
trate, 173;  pruning,  221-225; 
structure,  1 92-203 ;  underground, 
177;  use  in  propagation,  225- 
232. 

Stigma,  33,  33*. 

Siiink  Horn  Fungus,  392,  392*. 

Stinking  Smut,  395. 

Strpe,  386. 

Stipules,  235,  234*. 

Stock,  227. 

Stolons,  361. 

Stomata,  184;  function,  247;  location, 
248,  249;  structure,  247,  246.* 

Stoncworts,  329;  described,  332-335. 

Storage  tissue,  132. 

Strap-shaped  flowers,  25,  24*. 

Strawberiy  fruit,  79,  79*. 

Strengthening  tissue,  129,  128*,  129*, 
130*. 

Strobilus,  437,  439,  441,  447,  452. 

Strom  at  a,  371,  371*. 

Structure,  of  cells,  112-116,  114*, 
115*;  of  herbaceous  dicotyle- 
donous stems,  192-197;  of  leaves, 
242-252;  of  monocotyledonous 
stems,  187-192;  of  roots,  143- 
150;  of  stems,  182-203;  of  woody 
stems,  197-203. 

Struggle  for  existence,  517;  described, 
521. 

Strychnine,  283. 

Style,  34,  33*. 

Suberin,  276. 

Sugar  cane,  plants,  496,  496*;  propa- 
gation, 226,  228*,  229*. 

Sugar,  formation,  253;  kinds,  277; 
transformation  into  starch,  255. 

Sulphui',  as  a  soil  constituent,  157. 

Summer  spores  of  Rust,  397,  397*. 

Smidew,  272. 

Sui-vival  of  the  fittest,  517;  described, 
522. 

Suspensor,  466,  466*. 

Swamp  societies,  505. 

Sweet  Cherry,  212,211*. 

Sweethearts,  240*. 


Sweet  Potato  family,  490. 

Sweet  Potato,  propagation,  164,  164*. 

Symbiosis,  156. 

Sympetalae,  472;  families,  489-495. 

Synangium,  430. 

Systematic  Botany,  3. 

Tap-root,  139,  139*. 

Taphrina  pruni,  376. 

Tassel  of  Corn,  16*,  17*. 

Tartaric  acid,  285. 

Taxonomy,  3. 

Teleutospores,  397,  398*. 

Temperature,  in  relation  to  germina- 
tion, 89,  90;  in  relation  to  growth 
of  stems,  218;  in  relation  to 
photosynthesis,  260;  in  relation 
to  transpiration,  263. 

Temporary  roots,  103. 

Terminal  buds,  206. 

Terminal  flowers,  27,  27*. 

Tertiary  roots,  150. 

Testa,  55. 

Tetraspore,  329. 

Tetrasporic  plant,  328. 

Thalictrum,  467. 

Thallophytes,  292;  described,  296- 
405;  Algae,  296-329;  Bacteria, 
341-350;  BasicUomycetes,  382- 
404;  Blue-green  algae,  297-301; 
Brown  Algae,  318-324;  Flag- 
ellates, Diatoms,  and  Stone- 
worts,  329-335;  Fungi,  351-405; 
Green  Algae,  301-318;  Lichens, 
379-382;  Myxobacteria,  350; 
Red  Algae,  324-329;  Sac 
Fungi,  363-379;  Slime  molds, 
336-341;  Water  Molds,  353-363. 

Thorns,  272,  271*. 

Tillandsia,  162. 

Timothy  heads,  showmg  uuprove- 
ment,  529. 

Tissues,  general  view,  126-134;  ab- 
sorbing, 131,  131*;  conductive, 
130, 130*;  food-making,  132, 132*; 
meristematic,  126,  126*;  of 
leaves,  242-252;  of  roots,  146- 
150;  of  stems,  182-203;  protec 


INDEX 


585 


tivo,  127,  127*,  12S*;  roproduc- 
tivo,  133;  secretory,  133,  133*; 
storage,  132;  strengthening,  129, 
129*,  130*. 

Toadstools,  382;  described,  384;  de- 
structive, 388. 

Tobacco,  flower,  13,  12*;  leaf  arrange- 
ment, 239*. 

Tomato,  described,  491;  ovary,  36, 
34*,  36*;  pistil  natural  size,  37, 
37*. 

Torus,  12. 

Toxins,  123. 

Tracheae,  131. 

Tracheids,  130,  131*. 

Trailing  Arbutus,  490. 

Transpiration,  241;  advantages,  263; 
amount,  262;  conditions  affect- 
ing, 263;  dangers,  265;  described, 
260-269;  protection  against,  266. 

Traumatropsim,  152. 

Tree  Ferns,  427,  428*. 

Trichogyne,  327. 

True  Ferns,  426. 

Truffles,  366. 

Tryptophane,  281. 

Tube  nucleus,  42,  42*. 

Tuber,  179,  179*;  described,  181. 

Tuberales,  366. 

Tubular  Algae,  316-318. 

Tubular  flowers,  25,  24*. 

Turgor  pressure,  119. 

Twiners,  176. 

Typhaceae,  495. 

Tyrosin,  281. 

Ulothrix,  311,  311*. 
UmbeUiferae,  488. 
Umbel,  30,  30*,  32*. 
Uredinales,  396. 
Uredospore,  399. 
Urticaceae,  478. 

Vacuoles,  114,  114*,  115*,  156*. 
Variation,  517,  518*,  519*. 


Variation,  causes,  531;  continuous, 
525,  526*;   discontinuous,  527. 

Vascular  bundles,  130,  130*. 

Vascular  cylinder,  145,  146,  146*. 

Vaucheria,  317,  316*. 

Vegetative  reproduction,  409,  422. 

Vein,  244,  244*. 

Veinlets,  245,  244*. 

Velamen,  163. 

Venation,  245;  netted,  245,  244*; 
parallel,  245,  244*. 

Venter,  409,  410*. 

Venus  Flytrap,  273,  273* 

Vernal  habit,  170. 

Volva,  386,  384*. 

Volvocales,  307. 

Volvox,  305,  306*. 

Walnut  family,  474. 

Warmth,  as  an  ecological  factor,  501. 

Water,  as  an  ecological  factor,  50] . 

Water  Ferns,  426. 

Water  Mold,  353. 

Wliite  Heart  Rot,  389. 

White  Rust,  359. 

WiUow  family,  473. 

Wflt  Disease,  378. 

Wind,  as  an  ecological  factor,  502. 

Wing,  of  corolla,  24,  23*. 

Witches'  Brooms,  377. 

Wood  fiber,  129,  130,  130*. 

XanthophyU,  284. 
Xenia,  51,  51*. 

Xerophytic  societies,  509;  kinds,  510. 
Xylem,  130,  130*;  of  roots,  146,  149, 
146*,  149*;  of  stems,  188,  190*. 

Yeasts,  377. 

Zooglea  stage  of  Bacteria,  342,  343*. 
Zoospores,  302. 
Zygospore,  302. 
Zygote,  302. 
Zymase,  94,  285. 


